16 September 2019, 08:30 UTC
Anti-flares on the Sun – the eclipse season has begun on GOES satellitesSolar anti-flares on the flares activity graph.
Not very rare, and not less incomprehensible from it either – the appearance of ‘negative’ spikes of solar flares on the graphs are registered on the incoming data from space. This kind of dips on the images come out regularly, every year: in spring (March) and in autumn (September). However, often, they go unnoticed amid strong variations which are typical for solar flares’ graphs. These minor and short-term radiation dips simply do not get noticed sometimes. But, the same cannot be said about the incoming data, nowadays. Since amid extremely low solar activity the flares’ graphs are ‘degenerated’ into the almost straight lines, which is impossible, against this background, for occurring "anti-flares" to stay unnoticed.
The dips in radiation are not related to real solar activity in any way, but, do originate from the traits of the orbit of the GOES satellite which takes corresponding observations of the Sun. These observations are almost uninterrupted and what is happening now, is what is forcing to use the word ‘almost’. The matter of fact is that twice a year the device takes a position in the orbit where the line of its ‘sight’ to the Sun is covered by the Earth. This peculiarity is shared by all satellites located in the orbit named the geostationary orbit, where the devices rotate at the same speed as the Earth that is a period of 24 hours. Mainly, these are communication and meteorological satellites, for which is especially important that their rotation period is synchronized with the period of revolution of the planet. There are also two solar satellites amongst them, namely the GOES satellite (more precisely, several GOES satellites as it is a group of devices) and the SDO satellite, the one which provides the images of the Sun. The respective periods are called the Eclipse seasons. The schedule of these periods can be calculated theoretically; these, in particular, are published on the website of the GOES group of satellites - ospo.noaa.gov/Operations/GOES/eclipse.html. It is not difficult to understand that at the moment when the Earth is starting to cover the satellite from the sun, the signal from the Sun gradually decreases, and in the case where the Sun is fully obscured, till its complete disappearance. When the sun gradually leaves the earth ‘shadow’, the radiation is starting to rise until it has restored to its original value. That is how the ‘anti-flare’ occurring on the Sun.
According to the description provided on the GOES website, the eclipses for the GOES-R series of active space apparatuses are observed in average for 45 minutes before and 45 minutes after the local midday and start at about 45 days before and 45 days after the points of the equinox falling on 20-21 March (vernal equinox) and on 22-23 September (autumnal equinox). The actual schedule may significantly vary from the average. Such in this year, the first noticeable autumn dip was registered on the solar flares’ graphs on 2 September. Considering that the eclipse season should be symmetrical with respect to the equinox, its ‘anti-flares’ maximum will be reached on 23 September this year, and then their intensity will start to decrease. The last day of the season, when the dip will be still visible on the charts, will be on 13-14 October.
Since the satellite SDO is also located on the same orbit and is currently providing images of the Sun published on the website, the eclipses of the Sun by the Earth can be also periodically observed on the images of the Sun.
Laboratory of X-ray astronomy of the Sun, Lebedev Institute, Russia
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