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[气象图书馆] 电离层

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发表于 2009-2-4 21:57 | 显示全部楼层 |阅读模式
版主,你好,想请教几个问题——科普级的!
特别想学习“突发电离E、F层”(也有作:散见层、散乱层、偶发电离层)的形成机理与预报(可否):
因为这里已经离开对流层,接近空间天气的范畴,但是发现她又与对流层的许多气象级的现象有关。
所以问几个:高空中性风剪? 空间天气低层大气驱动源?对流层-电离层耦合?


    看来VOG非常好学,这边先支持鼓励一下h:35

    基于大气层的分类可以参考:http://www.21cma.net/thread-1556-1-8.html
    这边针对VOG提出的问题,先说一下电离层的分层。由于它影响到高频电波的传播,所以这点和VOG感兴趣的无线电非常有关。

1.D层
    D层是电离层最低的一层,离地球表面50至100公里。这里主要是波长为121.5纳米的赖曼-α氢光谱线的光电离一氧化氮。在太阳活动非常强烈时(一般超过50个黑子),硬X射线还可以电离空气中的氮气和氧气的分子。夜间宇宙射线造成一个剩余电离。这个层里离子对自由电子的捕获率比较高,因此电离效应比较低,因此它对高频无线电波没有影响。日间这里自由电子与其它粒子的碰撞率约为每秒1000万次。10MHz以下的电波会被D层吸收,随着电波频率的增高这个吸收率下降。夜间这个吸收率最低,中午最高。日落后这个层减弱非常大。D层最明显的效应是白天远处的中波电台收不到

2.E层
    E层是中层,在地面上100至150公里。这里的电离主要是软X射线和远紫外线对氧气分子的电离。这个层只能反射频率低于10MHz的电波,对频率高于10MHz的电波它有吸收的作用。E层的垂直结构主要由电离和捕获作用所决定。夜间E层开始消失,因为造成电离的辐射消失了,由于捕获在低处比较强,因此其高度开始上升。高空周日变化的风对E层也有一定影响。随着夜间E层的升高,电波可以被反射到更加远的地方。

3.ES层
       ES层也被称为偶现E层。它是小的、强烈电离的云,它可以反射频率在25至225MHz之间的电波。偶现E层可以持续数分钟到数小时不等,其形成原因可能有多种,而且还在研究中。夏季偶现E层出现得比较多,持续时间一般也比冬季长。电波的反射距离一般为1000公里左右。

4.F层
      F层离地面150至400公里。在这里太阳辐射中的强紫外线(波长10至100纳米)电离单原子氧。F层对于电波传播来说是最重要的层。夜间F层合并为一个层,白天分为F1和F2两个层。大多数无线电波天波传送是F层形成的。在白天F层是电离层反射率最高的层

    在电离层中阳光电离大气分子与离子重新捕获自由电子的过程平衡。一般来说高度越高,大气越稀薄,则电离过程约占上风。电离过程的主力是太阳及其活动。电离层内电离度主要由获得的太阳辐射所影响。因此电离层随周日和季节(冬季太阳高度角小,因此受到的辐射比较少)而变化。太阳活动主要随太阳黑子周期而变化。一般来说太阳表面黑子越多,太阳活动越强烈。除此以外随地球表面纬度的不同当地受到的太阳辐射强度也不同。耀斑和太阳风中的带电粒子可以与地球磁场相互作用,导致对电离层的扰乱。
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 楼主| 发表于 2009-2-4 22:04 | 显示全部楼层
上面说的是理想的理论情况,但是在实际情况中电离层不象上面所说的那样由规则、平滑的层组成。而其实电离层是由块状的、云一般的、不规则的电离的团或者层组成,这个叫“异常

    其中异常分为冬季异常和赤道异常

       夏季由于阳光直射中纬度地区的F2层在白天电离度加高,但是由于季节性气流的影响夏季这里的分子对单原子的比例也增高,造成离子捕获率的增高。这个捕获率的增高甚至强于电离度的增高。因此造成夏季F2层反而比冬季低。这个现象被称为冬季异常。在北半球冬季异常每年都出现,在南半球在太阳活动低的年度里没有冬季异常

    朝阳面电离层里的电流在地球磁赤道左右约±20度之间F2层形成一个电离度高的沟,这个现象被称为赤道异常(见下图)
    其形成原因如下:在赤道附近地球磁场几乎水平。由于阳光的加热和潮汐作用电离层下层的等离子上移,穿越地球磁场线。这在E层形成一个电流,它与水平的磁场线的相互作用导致磁赤道附近±20度之间F层的电离度加强。

电离层中还经常出现扰乱现象——X射线:突发电离层骚扰

太阳活跃时期强烈的耀斑发生时硬X射线会射击到地球。这些射线可以一直穿透到D层,在这里迅速导致大量自由电子,这些电子吸收高频(3-30MHz)电波,导致无线电中断。与此同时及低频(3-30kHz)会被D层(而不是被E层)反射(一般D层吸收这些信号)。X射线结束后D层电子迅速被捕获,无线电中断很快就会结束,信号恢复。

而地磁风暴是地球磁场暂时的、剧烈的骚扰。地磁风暴时F2层非常不稳定,会分裂甚至完全消失,在极地附近会有极光产生。

质子:极冠吸收

    耀斑同时也释放高能质子,这些质子在耀斑爆发后15分钟至2小时内到达地球。这些质子沿地球磁场线螺旋在磁极附近撞击地球大气层,提高D层和E层的电离。极冠吸收可以持续一小时至数日,平均持续24至36小时。

无线电应用

    电离层被用来反射和传送高频无线电信号。反射后的信号回到地球表面,可以再次被反射到电离层。

    电波可以使得电离层里的自由电子以同样的频率振荡。假如此时自由电子被捕获的话电波中的部分能量消失。假如电离层里自由电子的碰撞频率小于电波频率,而且自由电子密度足够高的话可以产生全发射。假如电波频率高于电离层内的等离子频率的话,则电子运动不够快来反射电波。在一个临界频率以下电离层可以垂直反射电波。
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发表于 2009-2-4 22:22 | 显示全部楼层
感谢得一塌糊涂!
     不知道怎么表达啦!!


—— 记录下来,下课好好学习去。
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 楼主| 发表于 2009-2-6 12:40 | 显示全部楼层
原帖由 VOG 于 2009-2-4 22:22 发表
感谢得一塌糊涂!
     不知道怎么表达啦!!

—— 记录下来,下课好好学习去。

       不客气h:22

       针对LS无线电在电离层中的应用,这边再补充说明一下无线电波在店里层中的传播机制。

    电离层由自由电子正离子负离子、分子和原子组成,是部分电离的等离子体介质。带电粒子的存在影响无线电波的传播,其机制是带电粒子在外加电磁场的作用下随之振动,从而产生二次辐射,同原来的场矢量相加,总的效果表现为电离层对电波的折射指数小于1。由于自由电子的质量远小于离子的质量,一般电子的作用是主要的,只要考虑电子就够了。但如电波频率较低而接近于离子的等离子体频率时,离子的影响也不能忽略。由于地磁场的存在,带电粒子也受它的影响,所以电离层又是各向异性的。

    电离层的形成和结构特性是受太阳控制的,因此它既随时间又随空间变化。在这样复杂的介质中,分析无线电波传播问题必须建立相对简化的物理模型并根据电波的频率采用相应的理论和方法。对于电离层电波传播,介质的折射指数是一个最根本的参数,实验证明相当有效。为人们普遍接受的磁离子理论表达的折射指数的公式称为阿普尔顿-哈特里公式,它是电离层电子密度和电波频率的函数,所以又被称为色散公式,而电离层则是一种色散介质。对于短波和波长更短的电波传播问题,可以采用近似的射线理论,对长波和超长波则一般需要采用波动理论,有时可将地面和电离层底部之间看作一个同心球形波导

   电离层的折射指数主要取决于电子密度和电波频率,电子密度愈大或电波频率愈低,折射指数愈小。因为电离层的折射指数小于1,电波在电离层中受到向下折射,在垂直投射的情况下,折射指数等于零时,电波不能传播,产生“反射”。在一定值的电子密度情况下,使折射指数为零的频率称为电波的临界频率,在地磁场的影响可以忽略时,这一频率就等于电子的等离子体频率。电离层的电子密度随高度的变化具有分层结构(见楼上),因此从地面向上传播的电波受到折射后传播路径逐步弯曲,最后转向地面;从而使地面上的远距离传播成为可能。较高频率的电波,穿透电离层的程度也较深,受折射影响偏离直线传播的程度则较小。电波频率超过某一数值时将穿透整个电离层而不被反射。在垂直投射时,对应这一频率的值就是电离层最大电子密度处的临界频率。在斜投射的情况下,也有一个大于上述垂直投射时临界频率的临界值,称为最高可用频率,用MUF表示,只有当使用的电波频率低于它时,电波才能返回地面。显然MUF与电波的投射角度有关,仰角愈小,MUF愈大,传播的距离也愈远。

   电离层对电波有衰减作用,称为电离层的吸收,主要是由电子与大气的分子或原子的碰撞所引起,所以吸收主要发生在低电离层(即D层)内。同时,在电波被电离层反射的区域,由于那里能量的传播速度较慢,经受吸收的时间较长,遭受的吸收也往往不能忽视。这一区域的吸收常被称为偏离区吸收;相对地在电波路径弯曲不大的那部分引起的吸收称为非偏离区吸收电离层对电波吸收的分贝数与频率的平方成反比,由于非偏离区吸收是主要的,所以在短波通信中多采用较高的频率或进行夜间通信。对于一定的传播电路、一定的信号形式和调制方式、一定的噪声和干扰水平、一定的发射功率和接收机性能,以及一定的通信质量要求,使用的频率有一个下限,称为最低有用频率,用LUF表示。
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 楼主| 发表于 2009-2-6 12:47 | 显示全部楼层
最后再介绍一下电离层中几种常见波段的传播原理   

    短波传播3~30兆赫为短波范围,它是实现电离层远距离通信和广播的最适当波段,在通常的电离层状况下,它正好对应于最高可用频率和最低有用频率之间。

   在地面两点之间,无线电短波段在电离层中的传播可以采取如下图所示的多种路径。假如从天线斜向发射一束电波,其频率大于发射点上空的临界频率。波束中仰角高的射线可以垂直或略微折射穿透电离层;仰角稍低的射线则如图中1~9经反射回到地面,称为天波。其到达地面的距离先是随仰角的变小而逐渐向发射点靠拢,如射线 1,2,3。到达距离最近的是射线4。此后当仰角继续降低时,射线到达地面的距离又逐渐增大。如射线5~9。由图可见,仰角降低的同时电波反射点也变低。射线4的仰角是个临界点,在4点以外区域中的任何一点可以看到有两条射线到达,一条的到达仰角较高,在空中经历的路程较长;另一条的到达仰角稍低,经历的路程稍短。前者称为高波,后者称为低波。射线4是高低波重合的特殊情形,在地面上4点附近能量集中,称为前沿聚焦。从4处到发射点之间,天波不能到达,而在靠近发射点处有沿地面传来的地波。天波和地波都不能到达的范围称为静区E层一次反射的最大传播距离约为2000公里,频率不能高于E层的2000公里MUF。F层一次反射的最大传播距离约为4000公里,频率不能高于F层的4000公里MUF。另一方面,一次电离层反射的传播距离也不能小于某一临界值,该临界值称为越距。天波能够经过电离层和地面的多次反射而传播到很远的距离,甚至可作环球传播。

   电离层短波传播的优点是可以用不很大的功率来实现远距离通信和广播。它的缺点是:因为电离层是色散介质,电离层传播的频带较窄,如不能传送电视;由于有多径效应,信号的衰落较大;太阳爆发会引起电离层暴和突然骚扰,这时电离层通信和广播可能遭受严重影响,乃至中断。

   中波传播 

    300千赫至3兆赫为中波波段,广泛用于近距离广播。在白天,由于 D层的吸收很大,天波很弱,中波传播主要靠地波;在夜间,由于D层基本上消失,中波可能被E层反射,传播至远达2000公里乃至更远处。因此相对地在近处地波较强,在远处天波较强,在中间某个距离范围内,天波与地波的场强相差不多,引起相互干涉的衰落现象。在夜间,E层不同反射次数的回波也可能引起干涉衰落。

   长波和超长波传播 

    对长波和超长波段 (30~300千赫和3~30千赫),一般地说,射线理论(即近似的几何光学方法)不再适用,必须用严格的全波理论来处理。对于几百公里以内的近距离传播,电离层的影响很小,天波可以不加考虑,而用一般的地波传播理论来处理。对于远距离的长波和超长波传播,其传播方式主要是地面与电离层低层边界之间的波导传播。这种传播方式主要用于远距离导航、标准时间信号的播送以及陆地对潜艇的通信。其优点在于:信号衰减较慢,传播距离较远,信号强度、传播速度和相位比较稳定。它们的稳定性受低电离层的高度和结构变化的影响,在日出日落时变化较大;在电离层突然骚扰时,信号会增强。

   长波和超长波还有另一种传播方式,即所谓哨声型传播。哨声是由雷电产生的频率在声频范围内的电磁脉冲信号,它的寻常波能基本上沿地球磁力线穿透电离层经磁层返回达地球另一侧,并从地面反射再沿原来的磁力线路径回到原先的半球,它甚至能往返传播多次。由于电离层色散效应,不同频率的成分按先高后低的次序到达,接收时可以听到口哨一样的声音,故称哨声。

   超短波的散射传播  

    超短波的频率范围从 30~300兆赫,300兆赫以上为微波波段。这两个波段的无线电波都将穿透电离层,因此它们主要是用于地面和空间飞行器之间的跟踪定位、遥测、遥控和通信联络。这时无线电波在穿透电离层的过程中或多或少地受到折射而影响到工程应用中的精度,因此要进行折射误差的修正。另一方面超短波的低端由于电离层中不均匀结构对电波的散射作用而使地面上点与点之间的传播成为可能,实际的电离层散射传播方式有如下几类:经过D层前向散射,适合于30~60兆赫,传播距离从1000~2000公里,但由于频带较窄,实用意义不大;利用流星余迹反射适用于40~80兆赫的间歇式通讯,距离可达2000公里;经过F层不均匀体散射,距离可达4000公里;利用偶发E层(Es层)反射,距离可达2000公里,频率可达80兆赫。当电路跨越极区时,可利用极光区电离气体反射。电离层中的随机不均匀结构对电波的散射能使它们的振幅、相位和射线到达角等都发生随机起伏,称为电离层闪烁。这种现象对于穿透电离层的无线电短波高端,乃至几千兆赫的微波波段都存在。
发表于 2009-2-6 18:32 | 显示全部楼层
再次表示诚挚的谢意!
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About the Ionosphere

Ionosphere

Relationship of the atmosphere and ionosphereThe ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the Thermosphere.

Contents
1 Geophysics
2 The ionospheric layers
2.1 D layer
2.2 E layer
2.3 ES
2.4 F layer
2.5 Ionospheric model
3 Anomalies to the ideal model
3.1 Winter anomaly
3.2 Equatorial anomaly
3.3 Equatorial electrojet
4 Ionospheric perturbations
4.1 X-rays: sudden ionospheric disturbances (SID)
4.2 Protons: polar cap absorption (PCA)
4.3 Geomagnetic storms
4.4 Lightning
5 Radio application
5.1 Mechanism of refraction
6 Other applications
7 Measurements
7.1 Ionograms
7.2 Incoherent scatter radars
7.3 Solar flux
7.4 Scientific research on ionospheric propagation
8 History
9 References
10 See also
11 External links

Geophysics:
The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km. It owes its existence primarily to ultraviolet radiation from the sun.

The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the ozone layer. At heights of above 80 km (50 miles), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere. In a plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.

Solar radiation at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing since photons at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision. At the same time, however, an opposing process called recombination begins to take place in which a free electron is "captured" by a positive ion if it moves close enough to it. As the gas density increases at lower altitudes, the recombination process accelerates since the gas molecules and ions are closer together. The point of balance between these two processes determines the degree of ionization present at any given time.

The ionization depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the sun. Thus there is a diurnal (time of day) effect and a seasonal effect. The local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. The activity of the sun is associated with the sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field.

The ionospheric layers:
Solar radiation, acting on the different compositions of the atmosphere with height, generates layers of ionization:

D layer:
The D layer is the innermost layer, 50 km to 90 km above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometre (nm) ionizing nitric oxide (NO). In addition, when the sun is active with 50 or more sunspots, hard X-rays (wavelength < 1 nm) ionize the air (N2, O2). During the night cosmic rays produce a residual amount of ionization. Recombination is high in the D layer, thus the net ionization effect is very low and as a result high-frequency (HF) radio waves aren't reflected by the D layer. The frequency of collision between electrons and other particles in this region during the day is about 10 million collisions per second. The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset, but remains due to galactic cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime.

During solar proton events, ionization can reach unusually high levels in the D-region over the high and polar latitudes. Such events are known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region. In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours.

E layer:
The E layer is the middle layer, 90 km to 120 km above the surface of the Earth. Ionization is due to soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). Normally this layer can only reflect radio waves having frequencies lower than about 10 MHz and has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. However during intense Sporadic E events it can reflect frequencies as high as 250 MHz The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer begins to disappear because the primary source of ionization is no longer present. This results in an increase in the height where the layer maximizes because recombination is faster in the lower layers. Diurnal changes in the high altitude neutral winds also plays a role. The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

This region is also known as the Kennelly-Heaviside Layer or simply the Heaviside layer. Its existence was predicted in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the British physicist Oliver Heaviside (1850-1925). However, it was not until 1924 that its existence was detected by Edward V. Appleton.

Es:
The Es layer or sporadic E-layer. Sporadic E propagation is characterized by small clouds of intense ionization, which can support radio wave reflections from 25 – 225 MHz. Sporadic-E events may last for just a few minutes to several hours and make radio amateurs very excited, as propagation paths which are generally unreachable, can open up. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs most frequently during the summer months with major occurrences during the summer, and minor occurrences during the winter. During the summer, this mode is popular due to its high signal levels. The skip distances are generally around 1000km (620 miles). VHF TV and FM broadcast DX'ers also get excited as their signals can be bounced back to earth by Es. Distances for short hop events can be as close as 500 miles or up to 1,400 (or more) for a long, single hop. Douple-hop reception over 2,000 miles is possible, too.

F layer:
The F layer or region, also known as the Appleton layer, is 120 km to 400 km above the surface of the Earth. It is the top most layer of the ionosphere. Here extreme ultraviolet (UV, 10–100 nm) solar radiation ionizes atomic oxygen. The F layer consists of one layer at night, but in the presence of sunlight (during the day), it divides into two layers, labeled F1 and F2. These F layers are responsible for most skywave propagation of radio waves, facilitating high frequency (HF, or shortwave) radio communications over long distances. They are thickest and most effective in refracting radio signals on the side of the earth facing the sun.

From 1972 to 1975 NASA launched the AEROS and AEROS B satellites to study the F region.[1]

Ionospheric model:
The atmospheric physics community contributes to the definition and maintenance of an ionospheric model: the International Reference Ionosphere, through a series of academic committees and conferences. As discoveries are made and generally accepted, the model is improved. (IRI85-6)

Anomalies to the ideal model:
The statements above assumed that each layer was smooth and uniform. In reality the ionosphere is a lumpy, cloudy layer with irregular patches of ionization.

Winter anomaly:
At mid-latitudes, the F2 layer daytime ion production is higher in the summer, as expected, since the sun shines more directly on the earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F2 ionization is actually lower in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity.

Equatorial anomaly:
Electric currents created in sunward ionosphere.Within approximately ± 20 degrees of the magnetic equator, is the equatorial anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain.

Equatorial electrojet:
The worldwide solar-driven wind results in the so-called Sq (solar quiet) current system in the E region of the Earth's ionosphere (100-130 km altitude). Resulting from this current is an electrostatic field directed E-W (dawn-dusk) in the equatorial day side of the ionosphere. At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within ± 3 degrees of the magnetic equator, known as the equatorial electrojet.

Ionospheric perturbations:
X-rays: sudden ionospheric disturbances (SID):
When the sun is active, strong solar flares can occur that will hit the Earth with hard X-rays on the sunlit side of the Earth. They will penetrate to the D-region, release electrons which will rapidly increase absorption causing a High Frequency (3-30 MHz) radio blackout. During this time Very Low Frequency (3 - 30 kHz) signals will become reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave, and thus dampen it. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out ends as the electrons in the D-region recombine rapidly and signal strengths return to normal.

Protons: polar cap absorption (PCA):
Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours.

Geomagnetic storms:
A geomagnetic storm is a temporary intense disturbance of the Earth's magnetosphere.

During a geomagnetic storm the F2 layer will become unstable, fragment, and may even disappear completely.
In the Northern and Southern pole regions of the Earth aurora will be observable in the sky.

Lightning:
Lightning can cause ionospheric perturbations in the D-region one of two ways. The first is through VLF frequency radio waves launched into the magnetosphere. These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto the ionosphere, adding ionization to the D-region. These disturbances are called Lightning-induced Electron Precipitation (LEP) events.

Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called Early/Fast.

In 1925, C. F. Wilson proposed a mechanism by which electrical discharge from lightning storms could propagate upwards from clouds to the ionosphere. Around the same time, Robert Watson-Watt, working at the Radio Research Station in Slough, UK, suggested that the ionospheric sporadic E layer (Es) appeared to be enhanced as a result of lightning but that more work was needed. In 2005, C. Davis and C. Johnson, working at the Rutherford Appleton Laboratory in Oxfordshire, UK, demonstrated that the Es layer was indeed enhanced as a result of lightning activity. Their subsequent research has focussed on the mechanism by which this process can occur.

Radio application:
DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. Thanks to the property of ionized atmospheric gases to refract high frequency (HF, or shortwave) radio energy, the ionosphere can be utilized to "bounce" a transmitted signal back to earth. The signal may then be reflected back into the ionosphere for a second bounce, or hop.

Mechanism of refraction:
When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio-frequency energy is given up to this resonant oscillation. The oscillating electrons will then either be lost to recombination or will re-radiate the original wave energy. Total refraction can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough.

The critical frequency is the limiting frequency at or below which a radio wave is refracted by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below:

where N = electron density per cm3 and fcritical is in MHz.

The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time.


where α = angle of attack, the angle of the wave relative to the horizon, and sin is the sine function.

The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by refraction from the layer.

Other applications:
The open system electrodynamic tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction.


Measurements:

Ionograms:
Ionograms show the virtual heights and critical frequencies of the ionospheric layers and which are measured by an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms.

Incoherent scatter radars:
Incoherent scatter radars operate above the critical frequencies. Therefore the technique allows to probe the ionosphere, unlike ionosondes, also above the electron density peaks. The thermal fluctuations of the electron density scattering the transmitted signals lack coherence, which gave the technique its name. Their power spectrum contains information not only on the density, but also on the ion and electron temperatures, ion masses and drift velocities.

Solar flux:
Solar flux is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a radio telescope located in Ottawa, Canada. Known also as the 10.7 cm flux (the wavelength of the radio signals at 2800 MHz), this solar radio emission has been shown to be proportional to sunspot activity. However, the level of the sun's ultraviolet and X-ray emissions is primarily responsible for causing ionization in the earth's upper atmosphere. We now have data from the GOES spacecraft that measures the background X-ray flux from the sun, a parameter more closely related to the ionization levels in the ionosphere.

The A and K indices are a measurement of the behavior of the horizontal component of the geomagnetic field. The K index uses a scale from 0 to 9 to measure the change in the horizontal component of the geomagnetic field. A new K index is determined at the Table Mountain Observatory, north of Boulder, Colorado.
The geomagnetic activity levels of the earth are measured by the fluctuation of the Earth's magnetic field in SI units called teslas (or in non-SI gauss, especially in older literature). The Earth's magnetic field is measured around the planet by many observatories. The data retrieved is processed and turned into measurement indices. Daily measurements for the entire planet are made available through an estimate of the ap index, called the planetary A-index (PAI).

Scientific research on ionospheric propagation:
Scientists also are exploring the structure of the ionosphere by a wide variety of methods, including passive observations of optical and radio emissions generated in the ionosphere, bouncing radio waves of different frequencies from it, incoherent scatter radars such as the EISCAT, Sondre Stromfjord, Millstone Hill, Arecibo, and Jicamarca radars, coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN) radars, and using special receivers to detect how the reflected waves have changed from the transmitted waves.

A variety of experiments, such as HAARP (High Frequency Active Auroral Research Program), involve high power radio transmitters to modify the properties of the ionosphere. These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and military purposes. HAARP was started in 1993 as a proposed twenty year experiment, and is currently active near Gakona, Alaska. There is concern among many members of the scientific community regarding the dangers involved in disturbing the ionosphere.

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20 MHz range. Coherent backscatter is similar to Bragg scattering in crystals and involves the constructive interference of scattering from ionospheric density irregularities. The project involves more than 11 different countries and multiple radars in both hemispheres.

Scientists are also examining the ionosphere by the changes to radio waves from satellites and stars passing through it. The Arecibo radio telescope located in Puerto Rico, was originally intended to study Earth's ionosphere.

History:
In 1899, Nikola Tesla moved from New York to Colorado Springs, Colorado, where he would have room for his high-voltage, high-frequency experiments. Upon his arrival he told reporters that he was conducting wireless telegraphy experiments transmitting signals from Pikes Peak to Paris.[2] Tesla's diary contains explanations of his experiments concerning the ionosphere.[3]

Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland (now in Canada) using a 152.4 m (500 ft) kite-supported antenna for reception. The transmitting station in Poldhu, Cornwall used a spark-gap transmitter to produce a signal with a frequency of approximately 500 kHz and a power of 100 times more than any radio signal previously produced. The message received was three dots, the Morse code for the letter S. To reach Newfoundland the signal would have to bounce off the ionosphere twice. Dr. Jack Belrose has recently contested this, however, based on theoretical and experimental work.[4] However, Marconi did achieve transatlantic wireless communications beyond a shadow of doubt in Glace Bay one year later.

In 1902, Oliver Heaviside proposed the existence of the Kennelly-Heaviside Layer of the ionosphere which bears his name. Heaviside's proposal included means by which radio signals are transmitted around the Earth's curvature. Heaviside's proposal, coupled with Planck's law of black body radiation, may have hampered the growth of radio astronomy for the detection of electromagnetic waves from celestial bodies until 1932 (and the development of high frequency radio transceivers). Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties.

In 1912, the U.S. Congress imposed the Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 MHz (wavelength 200 meters or smaller). The government thought those frequencies were useless. This led to the discovery of HF radio propagation via the ionosphere in 1923.

In 1926, Scottish physicist Robert Watson-Watt introduced the term ionosphere in a letter published only in 1969 in Nature:

We have in quite recent years seen the universal adoption of the term ‘stratosphere’..and..the companion term ‘troposphere’... The term ‘ionosphere’, for the region in which the main characteristic is large scale ionisation with considerable mean free paths, appears appropriate as an addition to this series.

Edward V. Appleton was awarded a Nobel Prize in 1947 for his confirmation in 1927 of the existence of the ionosphere. Lloyd Berkner first measured the height and density of the ionosphere. This permitted the first complete theory of short wave radio propagation. Maurice V. Wilkes and J. A. Ratcliffe researched the topic of radio propagation of very long radio waves in the ionosphere. Vitaly Ginzburg has developed a theory of electromagnetic wave propagation in plasmas such as the ionosphere.

In 1962 the Canadian satellite Alouette 1 was launched to study the ionosphere. Following its success were Alouette 2 in 1965 and the two ISIS satellites in 1969 and 1971, all for measuring the ionosphere.

References:
^ Yenne, Bill (1985). 'The Encyclopedia of US Spacecraft'. Exeter Books (A Bison Book), New York. ISBN 0-671-07580-2. p.12 AEROS
^ Tesla biography at magnetricity.com
^ Tesla, Nikola, "The True Wireless". Electrical Experimenter, May 1919. (also at pbs.org)
^ John S. Belrose, "Fessenden and Marconi: Their Differing Technologies and Transatlantic Experiments During the First Decade of this Century". International Conference on 100 Years of Radio -- 5-7 September 1995.
Corum, J. F., and Corum, K. L., "A Physical Interpretation of the Colorado Springs Data". Proceedings of the Second International Tesla Symposium. Colorado Springs, Colorado, 1986.
Davies, K., 1990. Peter Peregrinus Ltd, London. ISBN 0-86341-186-X Ionospheric Radio.
Grotz, Toby, "The True Meaning of Wireless Transmission of power". Tesla : A Journal of Modern Science, 1997.
Hargreaves, J. K., "The Upper Atmosphere and Solar-Terrestrial Relations". Cambridge University Press, 1992,
Kelley, M. C, and Heelis, R. A., "The Earth's Ionosphere: Plasma Physics and Electrodynamics". Academic Press, 1989.
Leo F. McNamara. (1994) ISBN 0-89464-804-7 Radio Amateurs Guide to the Ionosphere.
K.Rawer and Y.V.Ramanamurty (eds) (1 January 1986). "International Reference Ionosphere - Status 1985/86". Advances in Space Research 5 (10). ISBN 0-08-034026-1 (Publisher: Pergamon Press), ISSN 0273-1177.  

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发表于 2009-2-7 18:57 | 显示全部楼层
Ionosfera
Ionosphere for Radio-amateur & Scientific Purposes
Technical Group:
Ionospheric Effects & Activity Forecast

SDA objective
Services aiming at predicting and analysing radio-propagation through the ionosphere for the Radio-amateur community. The operational data collected in (and retrievable from) the Ionosfera website could serve the interested Radio-amateurs and Scientists to study this phenomena with a data source distributed all over the world.
Products

SDA description
The Pilot Project Ionosfera, proposed by AMSAT-Italia and selected by ESA, aims at supporting the Radio-amateurs and the Scientists in their activities relating to the Ionosphere.

The Ionosphere indeed is a direct product of the Sun-Earth interactions (hence a Space Weather topic), used by Radio-amateurs to achieve long-range radio-communication links.

However, Radio-amateurs in their daily (and nightly) activities face the major problem of link success unpredictability : because its dynamic behaviour is still not thoroughly understood, radio-propagation through Ionosphere refraction is still not reliable nor secure enough for practical use in applications such as for example, emergency communications in case of disasters.

Ionosfera in the frame of ESA's Space Weather Pilot Projects, will then provide two kinds of (free-of-charge) services :

? The provision of operational data on Radio-amateur communications and on Space Weather RF effects. Such information could be used later on by Scientists to consider the Ionosphere from another point of view and to improve, by integrating it with other available data, their Scientific models.

? A radio-communication prediction service based on several tools (number and complexity will be decided during the project development) allowing to predict/understand (maybe in real-time) long-range radio-communication links:

Amsat-Italia project: http://www.esa-spaceweather.net/sda/ionosfera/ 意大利
发表于 2009-2-7 19:06 | 显示全部楼层
ionosphere and magnetosphere
atmospheric science
Main
regions of Earth’s atmosphere in which the number of electrically charged particles—ions and electrons—are large enough to affect the propagation of radio waves. The charged particles are created by the action of extraterrestrial radiation (mainly from the Sun) on neutral atoms and molecules of air. The ionosphere begins at a height of about 50 km (30 miles) above the surface, but it is most distinct and important above 80 km (50 miles). In the upper regions of the ionosphere, beginning several hundred kilometres above Earth’s surface and extending tens of thousands of kilometres into space, is the magnetosphere, a region where the behaviour of charged particles is strongly affected by the magnetic fields of Earth and the Sun. It is in the magnetosphere that the spectacular displays of the aurora borealis and aurora australis take place. The magnetosphere also contains the Van Allen radiation belts, where highly energized protons and electrons travel back and forth between the poles of Earth’s magnetic field.

Ionosphere &raquo; Discovery of the ionosphere
Discovery of the ionosphere extended over nearly a century. As early as 1839, the German mathematician Carl Friedrich Gauss speculated that an electrically conducting region of the atmosphere could account for observed variations of Earth’s magnetic field. The notion of a conducting region was reinvoked by others, notably in 1902 by the American engineer Arthur E. Kennelly and the English physicist Oliver Heaviside, to explain the transmission of radio signals around the curve of Earth’s surface before definitive evidence was obtained in 1925. For some years the ion-rich region was referred to as the Kennelly-Heaviside layer.

The name “ionosphere” was introduced first in the 1920s and was formally defined in 1950 by a committee of the Institute of Radio Engineers as “the part of the earth’s upper atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves.” Much of the early research on the ionosphere was carried out by radio engineers and was stimulated by the need to define the factors influencing long-range radio communication. Subsequent research has focused on understanding the ionosphere as the environment for Earth-orbiting satellites and, in the military arena, for ballistic missile flight. Scientific knowledge of the ionosphere has grown tremendously, fueled by a steady stream of data from spacecraft-borne instruments and enhanced by measurements of relevant atomic and molecular processes in the laboratory.

Ionosphere &raquo; Layers of the ionosphere
Historically, the ionosphere was thought to be composed of a number of relatively distinct layers that were identified by the letters D, E, and F. The F layer was subsequently divided into regions F1 and F2. It is now known that all these layers are not particularly distinct, but the original naming scheme persists.

It appears that Edward V. Appleton, a pioneer in early radio probing of the ionosphere, is responsible for the nomenclature. Appleton was accustomed to using the symbol E to describe the electric field of the wave reflected from the first layer of the ionosphere that he studied. Later he identified a second layer at higher altitude and used the symbol F for the reflected wave. Suspecting a layer at lower altitude, he adopted the additional symbol D. In time, the letters came to be associated with the layers themselves rather than with the field of the reflected waves. It is now known that electron density increases more or less uniformly with altitude from the D region, reaching a maximum in the F2 region. Though the nomenclature used to describe the different layers of the ionosphere continues in wide use, the definitions have evolved to reflect the improved understanding of the underlying physics and chemistry.
发表于 2009-2-7 19:10 | 显示全部楼层
Ionosphere &raquo; Layers of the ionosphere &raquo; D region
The D region is the lowest ionospheric region, at altitudes of about 70 to 90 km (40 to 55 miles). The D region differs from the E and F regions in that its free electrons almost totally disappear during the night, because they recombine with oxygen ions to form electrically neutral oxygen molecules. At this time, radio waves pass through to the strongly reflecting E and F layers above. During the day some reflection can be obtained from the D region, but the strength of radio waves is reduced; this is the cause of the marked reduction in the range of radio transmissions in daytime. At its upper boundary the D region merges with the E region.

Ionosphere &raquo; Layers of the ionosphere &raquo; E region
The E region is also called Kennelly-Heaviside layer, named for American electrical engineer Arthur E. Kennelly and English physicist Oliver Heaviside in 1902. It extends from an altitude of 90 km (60 miles) to about 160 km (100 miles). Unlike that of the D region, the ionization of the E region remains at night, though it is considerably diminished. The E region was responsible for the reflections involved in Guglielmo Marconi’s original transatlantic radio communication in 1902. The ionization density is typically 105 electrons per cubic centimetre during the day, though intermittent patches of stronger ionization are sometimes observed.

Ionosphere &raquo; Layers of the ionosphere &raquo; F region
The F region extends upward from an altitude of about 160 km (100 miles). This region has the greatest concentration of free electrons. Although its degree of ionization persists with little change through the night, there is a change in the ion distribution. During the day, two layers can be distinguished: a small layer known as F1 and above it a more highly ionized dominant layer called F2. At night they merge at about the level of the F2 layer, which is also called the Appleton layer. This region reflects radio waves with frequencies up to about 30 megahertz; the exact value depends on the peak amount of the electron concentration, typically 106 electrons per cubic centimetre, though with large variations caused by the sunspot cycle.
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