A&A 387, L25-L28 (2002)
A. V. Arkhipov (O. V. Arkhypov)
Institute of Radio Astronomy, Nat. Acad. Sc. of Ukraine, Chervonopraporna 4, 61002 Kharkiv, Ukraine
Received 5 December 2001 / Accepted 2 April 2002
The satellite Ganymede sometimes occults the sources of the Jovian decameter radiation (DAM) associated with Io magnetic field line. The basic parameters of Ganymede occultations are calculated for 1990-2010. One of these events is found to coincide with a Io-A radio storm, which has been recorded in Nancay Observatory on 17 April 1994. In spite of the difficulty to identify the satellite shadow on sporadic DAM, the ratio of frequency emitted to calculated gyromagnetic frequency of electrons in the source is tentatively estimated as . Formally, this limit contradicts the present generation theories where in the DAM source is much closer to 1. Hence, improvements to the magnetic model (VIP4) or of the distortion of the Io flux tube are needed. Two possible shadows of the satellite are tentatively identified on the DAM frequency-time spectrogram. Multiple occultations are indeed possible in the Alfven wave model of Io-DAM interaction, and the lead angle of the emitting field line is not well known. That is why the tentative location of the radio source is made for both variants.
Key words: planets and satellites: individual - occultations - magnetic fields - radiation mechanisms: non-thermal
It has been shown that the Galilean satellites can eclipse the sources of the Jovian decameter radiation (DAM) (Arkhipov 1997, 2001). Such occultations could help to precisely localize the regions of DAM generation and reveal the fine structure of the DAM sources. Moreover, the ratio of emission frequency to local cyclotron frequency of electrons in the source could be measured and used for evaluation of DAM theories.
The occultation method had been applied to the Jovian radio emissions only during the Galileo mission, at short distances and low frequencies (5.6 MHz; Kurth et al. 1997). In this way, new information could be obtained from Earth based observations.
That is why archive searches for DAM eclipses as well as new observations are very desirable. For this task the basic parameters of Ganymede occultations are calculated in this article.
In accordance with present ideas on the radio emission generation, the following model is assumed for occultation calculations.
Jovian decameter radio emission is generated in fast extraordinary mode just above the local cyclotron frequency of electrons ( ) (see e.g. Zarka 1998). Even if , refraction probably becomes negligible at a short distance from the source, so that straight line propagation is an acceptable approximations for simplification of calculations. Of course, f could be significantly above (it is unknown a priori). However, our approximation is still a useful asymptotic marker for prediction and practical search for occultations.
The Io-controlled emission is generated along or close to the magnetic field line connected with Io (Io flux tube - IFT; Fig. 1) (Carr et al. 1983). This IFT is calculated as an undisturbed magnetic line according to the VIP-4 model (Connerney et al. 1998). Of course, some delay of Io-Jupiter interaction must be taken into consideration. Thus the radio emitting field line is traced, which is formally connected with "effective''position of Io in the satellite orbital plane, at jovigraphic longitude , where: is the true Io longitude; and is the lead angle. Here the corrections are supposed with the two alternative methods:
(a) as approximation of difference between orbital longitudes and the Io effective position, deduced from UV observations of IFT footprints (Clarke 1996): (north); (south). Uncertainty on the lead angle is of the order of several degrees.
(b) according to the analysis of DAM radiation pattern (Queinnec & Zarka 1998), (north) or (south) has been adopted.
This dual strategy is reasonable, because the lead angle may be different in radio and UV, as the emitting electrons may not be exactly the same populations. As a result, the position of an Io-controlled DAM source is calculated as a point with on the magnetic line, which is connected with Io's effective position.
The geocentric, differential coordinates of Jovian satellites are
calculated with interpolation of ephemerides from Bureau des
longitudes (www.bdl.fr/ephem/ephesat/satformbis.html). Although
the errors of calculated positions are 0.001
is the equatorial radius
of Jupiter), the real precision of eclipse prediction is limited
by the lead angle uncertainty. Practically the error in visible
coordinate of source is about the diameter of Ganymede (0.074
|Figure 1: Scheme of an occultation of the Io flux tube by Ganymede.|
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To plan and analyze the observations, all IFT occultations by Ganymede have been calculated for 1990-2010 (see Table 1). Sometimes the Ganymede's centre occults IFT only with the UV lead angle, while it is impossible with the radio angle and vice versa. The following information is tabulated there for both versions (a) and (b) of lead angle: the date of occultation (day/month/year); the universal time of the event (UT; hour, min.); the frequency of the shadow center in dynamical spectrum of DAM (MHz); the central meridian longitude of Jupiter at occultation (CML, degree in system III 1965); the Io phase at occultation ( , degree;); the hemisphere of occulted source (N- north; S- south); the geographical longitude ( , degree); and latitude ( , degree) of sub-Jovian point on the Earth at occultation.
To estimate the visibility of Jupiter for any observatory, the
zenith angle of Jupiter could be calculated
Obviously, such occultation is observable when a satellite
eclipses the active DAM source, emitting towards the Earth.
According to the maps of DAM occurrence for right- and left-hand
polarizations separately (Carr & Desh 1976; Boudjada &
Genova 1991), the most promising occultations are
|Figure 2: Probable IFT occultations on the DAM dynamical spectrum of 17 April 1994 (right hand polarization). The predicted shadow contours with and a) or b) are shown as the light ellipsoids on the left panel. The candidates in the Ganymedes shadow are shown on the right panel: c) and ; d) and .|
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Unfortunately, no Jupiter observations were made on predicted dates with the UTR-2 radio telescope of the Institute of Radio Astronomy (Kharkov, Ukraine). The synoptic dynamic spectra of DAM from the archives of Nancay Observatory (http://www.obs-nancay.fr/jupiter) have thus been used for the occultation search. As Ganymede occults some part of an emitting magnetic field line, the corresponding interval of frequencies must be shadowed in the DAM dynamical spectrum. Thus an elongated dark spot is formed in the spectrum, and this could be calculated. The typical dimensions of such a spot are 10 min and 9 MHz.
All corresponding Nancay spectra have been analysed (28/01/1994; 17.04.1994; 24/04/1994; 28.09.1994; 12.02.1995; 25.08.2000). Only on 17 April 1994 an Io-A storm (with right hand polarization) was recorded just in the predicted time interval of occultation. Both calculated contours of the Ganymede shadow, with the different lead angles (a) and (b), overlap the L-emission (Fig. 2, left). Of course, one possible explanation is the existence of several simultaneous radio sources, out of which only one is occulted by the satellite shadow. Conversely, there are two possible identifications of Ganymedes shadow above the Io-storm (Fig. 2, right). The most suitable ellipsoids are calculated with and , and for (c) and (d) contours respectively. The search for interference fringes which should exist along the shadow borders (Arkhipov 2001) is impossible with the low resolution of the published spectrum.
In spite of the recognition problem, the spectrum of 17 April 1994
is of interest. Thus,
any identification of the shadow in the most probable range
Formally, this limit contradicts the hypothesis
in the DAM source is much closer to 1.
It is now believed (Zarka 1998) that DAM emission is
produced close to the X mode cutoff at
It is difficult to explain this result in terms of exotic lead angle ( or ). Apparently, the derived from magnetic field model could be underestimated. However, the gyromagnetic frequency, calculated using VIP4 and previous O6 GSFC models, has a quadratic difference of only 1.4 MHz along the Io magnetic shell at planetocentric distance of the occultation ( ). Hence, the standard error on calculated is <1 MHz with VIP4, while the storm border exceeds the calculated by >2.3 MHz. Another explanation of this discrepancy may be due to the additional magnetic field distortion by the electrical current which flows along the Io flux tube.
It has been strongly argued that Io-B DAM cannot arise from the instantaneous IFT only (Leblanc et al. 1994; Queinnec & Zarka 1998). It was proposed that the arcs of Io-controlled DAM are caused by a pattern of field-aligned currents, which are separated in longitude. These currents are carried by the Alfvenic disturbances as they bounce between the northern and southern ionospheres and the Io torus edge (e.g.: Bagenal & Leblanc 1988). Apparently, there are two arcs in the Io-A storm of 17 April 1994. Hence, a pair of radio sources could cause two shadows in the dynamic spectrum. This also justifies why both possible shadows are worth consideration.
The shadows border point with lowest frequency could be used
for localization of the occulted radio source without any
dependence upon the magnetic model. The universal time and
frequency of this point is found for (c) ellipsoid by parabolic
approximation of the storm border:
MHz (arrowed in Fig. 2). The tangential coordinates of the radio source with
this frequency are:
I would like to express many thanks to Dr. Ph. Zarka, Dr. J. E. P. Connerney, Dr. J. H. Lieske, Dr. H. O. Rucker for their help with the literature and consultations. I also wish to thank Dr. L. Fleming for reading and commenting on the manuscript.