Adv. SpaceRe.,.Vol. 11,No.1, pp. (1)165—(1)167,1991 0273—1177/91$0.00+ .50Printed in Great Britain. All rights reserved. Copyright © 1991 COSPAR

EUV SOLAR EMISSION MEASUREMENTSBY PHOBOS SPACECRAFT

T. V. Kazachevskaya,L. L. Bukusova,D. A. Gonyukh,A. I. Lomovsky and Yu. N. Tsygelnitsky

Fedorov Institute of Applied Geophysics,Glebovskaya20-b, 107258,

Moscow, U.S.S.R.

ABSTRACT

Data on the extreme TV solar emission were obtained on a daily basis withinthe framework of the International Project PHOBOS. The measui~ements wereperformed using the SUPR (solar ultraviolet radiometer) equipment for recor-ding the solar emission flux at .. < 130 nm based on the thermoluminescenttechnique. The technique provides absolute measurements within the abovespectral range using CaSO4(Mn) thermophosphorus /1/.

INSTRUMEI’ITS

The energy of short wave solar radiation is stored during a certain time pe-riod by a thermoluminescent phosphor CaSO4(Mn) applied on a ceramic plate.Subsequent heating of the plate causes the phosphor to re—radiate the ener-gy stored in the visible (green) region at 1.. 500 nm.

A photo—multiplier and electronic scheme transform the light thermolumines-cent flux into a signal which is transmitted to a telemetric system.

Various filters preceeding the phosphors single out individual spectrum re-gions during UV radiation intensity measurements. The energy stored by thedetector is proportional within individual spectrum intervals to the totalradiation flux during the exposure.

The dynamic range of the measurements of solar UV radiation intensity is0.1,30 erg/cm~s.

The instrument’s inner operation cycle includes interrogating 24 detectors:detectors without filters, measuring the total UV radiation flux, and detec-tors with filters (MgP~, thin aluminium foil, thin Maylar film).

Every two minutes the instrument’s operation cycle allows one to obtain in-formation on the total solar emission flux at .?,(130 rim, measure H L~ lineemission (3t.. =121.6 rim) and emission at ?..< 22 rim and ?~< 12 rim, as well asmonitor the detector background and instrument sensitivity.

In 1984—1985 jointly with VNIIOPI*, a technique was developed on absolutecalibration by synchrotron radiation /2/ and by secondary references. A se-ries of observations are carried out to determine the relative sensitivityof the equipment in the range 100—140 rim and an absolute calibration is per-formed by resonance luminosity lines .2~ 115.6, 123.6 and 129.6 rim. The ab-solute calibration error does not exceed 15% of the measured value.

The SUPR equipment calibrated beforehand was installed aboard the SC Phobos1,2 and launched on 7 and 12 July 1988, respectively. The equipment beganworking on the first day the spacecraft (SC) was injected into the orbit.The information from.SC Phobos 1 was received up to 29 August 1988, and thatfrom SC Phobos 2 — over the whole flight route Earth—Mars and near Mars up

(1)166 T. V. Kazachevskayaet a!.

MEASUREMENTRESULTS

The short wave solar flux was measuredthroughout the whole flight routefrom the Earth to Mars and Phobos orbits. Data on EUV solar flux in theMar8 orbit were first obtained by Phobos 2 SC. Observations fell in a peri-od close to the solar activity maximum. A rather high level of UV solaremission was measured: EUV emission flux C .2130 nm), reduced to 1 A.U.,was equal to “- (9—12.5) erg/cm2s and (6—8) erg/cm~s in the H L~—line.Significant variations in the short wave emission flux were recorded, whichwere related to solar activity and flares.

Of particular interest are the large solar flares in March 1989 recorded bySUPR equipment aboard the SC Phobos 2, which was then orbiting Mars atabout 9.7 thousand km and at about 9—18 thousand km from Phobos. At thattime the solar EUV emission flux, reduced to 1 A4t., (with a measurement er-ror not higher than 15%) was -..‘(11.5—12.5) erg/cm’s at 2~<130 rim and (7—8) erg/cm’~s In the H L~line.

Table 1 presents dates and times (UT) of solar flares recorded by the SUFRequipment, the EUV emission flux increase (per cent) relative to the undis-turbed level, as well as the optical and X—ray class of flares observed atthe time near the Earth by GOES—7/3/.

TABLE 1 EUV Flares Recorded in March 1989 by SC Phobos

Date Time of Recording EUV Emission X—Ray Class Opticalby the SC Equip— Increas~e Classmerit, UT (2,.<130 rim)

8.03.89 18h 31m — 18h 36m 11 percent — —

19h 08m — 19h 12m 16.5 percent M4.6 Sub.

9.03.89 18h 37m — 18h 40m 10 percent — SF19h 16m — 19h 18m 11.5 percent M1.2 TB

10.03.89 19h 08m to the end double X4.5 3Bof the measurements increase

11.03.89 from 18h 24m 16 percent M1.2 iN

The SUPR detector signals did not exceed background values when the “quiet”Sun was observed using aluminium and Maylar filters. The filter capacitytaking account of the photocathode spectral sensitivity allows solar flareobservations in the region .2~. < 10 rim, beginning from M4—M5class. Thus, forthe flares on 8 March 1989 at 19 h 08 miii UT and on 10 March 1989 (seeTable 1), a large increase in the signal from the filter—free detector at.~.<i30 rim resulted from a considerably increased emission in the X—ray

spectral region. Measurements show the emission In the latter flare (withinthe ranges of 0.1—10 rim and 0.1—22 rim) to increase by.-.-2.5 and ~-‘-7—8times,respectively, as compared to the “quiet” Sun. ~he emission intensity calcu-lated w~th respect ~o 1 A.U. was ~ 1— ~2x10~’ quantum/cm2s and IO,,i...2 2 ~1.35x10 quantum/cm s while H L,~—lifie l~tensity increased by —19%.

The difference between X—ray solar fluxes recorded with SUPR and ones typi-cal for flares of X4.5 class /3/ can be attributed to the fact that thesewere measured during the initial phase of the flare development (before19 h 13 mm UT) and, unfortunately, failed to include the maximum.

The angle between the Sun, the Earth and the SC at Mars is 570• This meansthat the SUPR equipment was in a position to see flares related to groups ofspots 4—5 days before their appearance at the Sun side visible from theEarth. This can refer to the phenomenon observed on 8 March 1989 at 18 h30 mm UT.

n,. 4 4- ~.n,,1 ~ ~ ~,n+~A +1....+ ~ ~ n~ ~ 4 ~ ~‘TT5T

EUV EmissionObservations (1)167

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