Data gaps due to missing telemetry: ~5.5 h.

Note, that the link to the catalogue page  is now: http://www.cbk.pan.wroc.pl/resik_catalogue.htm.

A Long CORONAS-F Day Started Again

One of the peculiarities of the CORONAS-F spacecraft is its orbit (cf. W42, 2002). The orbit has been designed to be semi Sun- synchronous. This means that two times a year, the spacecraft (S/C) leaves Earth shadow and in this way the periods starts when uninterrupted solar illumination lasting several days is at the disposal. For RESIK it means that the active observing time is increased, with the instrument being OFF during passages of radiation belts and/or South Atlantic Anomaly (SAA) only. Just recently, on February, 10^th such a long day started again. It will last for about ten days.

In Figure 1, we present a plot showing the duration of the S/C night over the entire 2002 and 2003 years. It is seen from the plot, that we will get another chance(s) to observe the Sun for at least a few days.

Figure 1. Plots of the spacecraft (S/C) night duration in the past ( grey area) and the future. The upper plot covers two years from the beginning of 2002 till the end of 2003.  Pronounced dips indicate long S/C days lasting many weeks. In the lower panel, the present „long day” is shown in more details. It is seen that the first forthcoming night will show-up by the end of February 2003.

The presented plot has been calculated based on present values of the orbital parameters and may slightly deviate from reality as the time for prediction increase. Another aspect of the predictions shown is that the duration of the night portion of an orbit represents the „optical” night i.e. the time when the Earth occults solar illumination in the visible range. As already indicated in Weekly#51 (W51, 2002), the X-ray radiation of celestial sources is completely absorbed by the higher regions of the Earth atmosphere ~80 – 90 km above the ground. Therefore the Earth radius appears larger by the same amount when the X-ray night are to be considered. A direct result of this fact is that the X-ray night is still observed on the RESIK X-ray lightcurves at the times when no formal night is seen by the optical sensor. A slow shift of the orbital plane relative to Sun-Earth spatial configuration causes this effect to be presented for initial ~48 hours of the long satellite day.

An illustration of this can be found on a series of cuts  taken from the RESIK Catalogue.

	The orbit with the „last” night period seen by the optical sensor.
	After  ~40 hours, The X-ray nights are clearly visible on the RESIK lightcurve.
	After 44 hours, The X-ray night is barely seen on the record, and the dip disappears around 48 hours into the long day.

Figure 2. Top portions of RESIK catalogue pages for few recent days showing disappearance of the nights as the spacecraft entered its present „long day”. Worth noting is the delay (by 2 days) of the X-ray night disappearance relative to „optical day”.  The moments of  X-ray nights are indicated by the arrows. See the text for more explanation. The Legend to the Catalogue page has been given in Weekly#04 (W04, 2003).

It is worth to note that each long CORONAS-F day has its particular characteristics as the orbital plane slowly precess.

Report prepared by:  Janusz Sylwester js@cbk.pan.wroc.pl, Barbara Sylwester bs@cbk.pan.wroc.pl and Witold Trzebinski  wt@cbk.pan.wroc.pl.

The report presented  has been done in "real time" and so it may contain jargon, blunders, or trivialities. We do not have also an English native speaker in our Wroclaw group! We would be happy to discuss problems mentioned above in more details  if necessary.
RESIK data are in the open public domain and can be requested from: http://surfwww.mssl.ucl.ac.uk/surf/data_request.html.

Previous RESIK_weekly notes are in the archive: http://www.cbk.pan.wroc.pl/resik_archive.htm
 

Page made on 14 February 2003 by:  Anna Kepa ak@cbk.pan.wroc.pl and Jarek Bakala jb@cbk.pan.wroc.pl