When a solar flare occurs, EUV (Extreme Ultraviolet) and X-ray radiation is emitted from the Sun. When the radiation reaches the Earth's atmosphere (after ~8 minutes),
it can increase the ionization of the D and E regions in the ionosphere, enhancing VLF propagation. Gamma-rays from powerful GRBs can also have a similar effect on the ionosphere.
The enhancement of the VLF sky-wave can interfere with the ground-wave, causing constructive or destructive interference depending on the phase. If strong enough, this can be observed
in the plot of received power vs time.
The SID chart can plot multiple series, allowing different signals from different transmitters to be monitored.
This can be useful as SIDs can be localized to specific regions in the atmosphere, thus not all signals may be affected.
To help determine the cause of a SID, addtional data can be plotted from a variety of sources:
* the chart can plot X-ray data from the GOES satellites, to allow visual correlation of spikes in the X-ray flux measurement with spikes in the VLF power measurements,
* it can display images and video from the Solar Dynamics Observatory at EUV wavelengths, which may visually show the solar flare,
* it can display GRB events on the chart, measured by satellites such as Fermi and Swift,
* it can display solar flare events detected by the STIX X-ray instrument on the Solar Orbiter satellite,
* it can display proton flux measured by the GOES satellites,
* it can control the time in a 3D [Map](../../feature/map/readme.md), to see the corresponding effect on MUF (Maximum Usable Frequency) and foF2 (F2 layer critical frequency).
The SID feature can record power from any RX channel plugin that has a channelPowerDB value in its channel report, so can be used for recording and plotting power vs time for purposes other than SID monitoring.
When unchecked, data from [Channel Power](../../channelrx/channelpower/readme.md) plugins is displayed on a separate chart to other data such as X-ray and proton flux and GRBs.
Check to display a legend on the chart. When unchecked the legend will be hidden. You can click on items in the legend to temporarily hide and then show the corresponding series on the chart.
The position of the legend can be set in the Settings Dialog.
When checked, displays imagary from NASA's SDO (Solar Dynamic Observatory) and ESA/NASA's SOHO (Solar and Heliospheric Observatory) satellites.
SDOs images the Sun in a variety of UV and EUV wavelengths. SOHO shows images of the solar corona. The images are near real-time, updated every 15 minutes.
* AIA 94 <20> to 1700 <20> - The AIA (Atmospheric Imaging Assembly) images the solar atmosphere at multiple EUV (Extreme Ultraviolet) and UV (Ultraviolet) wavelengths:
* MHI Magnetogram - HMI (Helioseismic and Magnetic Imager) Magnetogram shows the magnetic field in the photosphere, with black and white indicating opposite polarities.
* MHI Intensitygram - Brightness in a visible light band (6173 <20> - Red - Iron spectral line), useful for observing sun spots.
* Dopplergram - Shows velocities along the line-of-sight.
* LASCO (Large Angle Spectrometric Coronagraph) shows solar corona. C2 shows corona up to 8.4Mkm. C3 shows corona up to 23Mkm.
When checked, opens a [Satellite Tracker](../../feature/satellitetracker/readme.md) feature and sets it to display data for the GOES 16, GOES 18 and SDO satellites.
The position and tracks of the satellites will then be visible on a [Map](../../feature/map/readme.md) feature.
When clicked, the chart X-axis is automatically scaled so that all power data is visible. When right-clicked, autoscaling of the X-axis will occur whenever new data is added to the chart.
When clicked, the chart Y-axis is automatically scaled so that all power data is visible. When right-clicked, autoscaling of the Y-axis will occur whenever new data is added to the chart.
Displays/sets the current start time of the chart (X-axis minimum). It's possible to scroll through hours/days/months by clicking on the relevent segment and using the mouse scroll wheel.
Displays/sets the current end time of the chart (X-axis maximum). It's possible to scroll through hours/days/months by clicking on the relevent segment and using the mouse scroll wheel.
Specifies the date and time for which SDO imagery should be displayed. Images are updated every 15 minutes. The date and time can also be set by clicking on the chart.
Select a Map to link to the SID feature. When a time is selected on the SID charts, the [Map](../../feature/map/readme.md) feature will have it's time set accordingly.
In order to check that a peak in the spectrum is a real VLF signal, you can:
* If using a magnetic loop or other directional antenna, rotate it and make sure the amplitude varies, as mag loops should have a null orthogonal to the plane of the loop.
* Check that the signal has diurnal variation (it should vary with the time of day, due to the changes in the ionosphere).
* Check with online lists of VLF signals (E.g. https://sidstation.loudet.org/stations-list-en.xhtml or https://www.mwlist.org/vlf.php). A number of these are plotted on the [Map](../../feature/map/readme.md) feature.
Occasionally, the X-ray flux data may drop to 0. This is typically when the GOES satellite is in eclipse (The Earth or moon is inbetween the satellite and the Sun).
SIDs are most likely to be detected when it's day time in the path between the signal source and receiver, as at night, the atmosphere is shielded from the X-rays by the Earth.
Also, as the D layer in the ionosphere essentially disappears at night, the received power is not as constant as during the day.
<h2>Codecs</h2>
On Windows, you may need to install an mp4 codec to view the SDO videos. Try [K-Lite Codecs](https://www.codecguide.com/download_k-lite_codec_pack_basic.htm).
<h2>Attribution</h2>
X-Ray and proton data is from [NOAA](https://www.swpc.noaa.gov/products/goes-x-ray-flux).
Solar images are from [SDO | Solar Dynamics Observatory](https://sdo.gsfc.nasa.gov/).
Corona images are from [SOHO](https://soho.nascom.nasa.gov/home.html).
GRB data is from [GRBweb](https://user-web.icecube.wisc.edu/~grbweb_public/index.html).
Solar flare data is from [Solar Orbiter STIX Data Center](https://datacenter.stix.i4ds.net/).
