6.2 Study of the solar wind influence
6.2.1 Magnetopause crossings
A first method to estimate the solar wind conditions consists in identifying the mag-netopause position. At the magmag-netopause, the magnetic pressure and the thermal pressure approximately balance the dynamic pressure of the solar wind (see Section 1.2.2). Hence, the magnetopause location gives an indication on the solar wind ram pressure. As il-lustrated in Section 6.1.2, the Cassini magnetopause crossings leave a signature in the low-energy electron spectrogram as well as in the magnetic field. Using CAPS/ELS and MAG data, Pilkington et al. (2015) listed the magnetopause crossings from Saturn Orbit Insertion to October 2010 and from May 2012 to February 2013. In total, 1514 cross-ings are included in this list. The authors have also estimated for each crossing the solar wind dynamic pressure using two different methods. In the first method, the magneto-spheric pressure is calculated at the magnetopause based on magnetometer and plasma (electron and suprathermal ion) data and the ram pressure is estimated assuming equi-librium between the two pressures. The second method consists in fitting an empirical magnetopause model through each crossing and finding the ram pressure satisfying the fit.
The pressure calculated by the first method is more realistic and gives more informa-tion about the dynamics occurring at the magnetopause at the time of the crossing. The second method has the advantage to be more stable to magnetopause instabilities such as
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Figure 6.3: Distribution of the Cassini magnetopause crossings located within 2 RSfrom the equatorial plane and provided by Pilkington et al. (2015) (covering the period from Saturn Orbit Insertion to late 2010 and from mid-2012 to early 2013). The color indicates the estimation of the solar wind dynamic pressure at each crossing using the balance between the dynamic pressure and the internal magnetospheric pressure (left panel) and an empirical magnetopause model (right panel). Titan’s orbit at 20.3 RSis drawn.
Kelvin-Helmholtz instability. In Figure 6.3, all the magnetopause crossings provided by Pilkington et al. (2015) and located within 2 RSfrom the equatorial plane are represented by dots with a color code giving the estimated solar wind dynamic pressure using the first method (“balance method”, left panel) and the second method (“model method”, right panel).
The estimation of the solar wind dynamic pressure obtained through the “balance method” can differ from the estimation coming from the “model method”, especially when instabilities are present at the magnetopause. A comparison between both methods is given in Figure 6.4. It is observed that the pressure estimated by the balance method is generally higher than the pressure given by the model method.
For the investigation of the solar wind effect on the hourly electron pulsations, a se-lection of 70 Cassini orbits containing at least one magnetopause crossing and one QP60 event has been achieved. These orbits are represented in Figure 6.5 on an equatorial pro-jection (left) and on a noon-midnight meridian propro-jection (right). In the next analyses, in order to have a reliable number of pulses occurring on a daily basis, only the pulsa-tions observed before the first magnetopause crossing on the outbound trajectory and the ones observed after the last magnetopause crossing on the inbound trajectory have been considered, constituting a list of 1142 pulses.
In the present analysis, the estimated ram pressure is based on the measurements at the magnetopause crossings. Therefore, only the electron events located very close to the magnetopause can be taken into account in order to have a reliable investigation of the solar wind correlation. The solar wind conditions at the magnetopause may have significantly changed for the events detected much earlier or later than the crossing time.
6.2 Study of the solar wind influence
Balance method solar wind dynamic pressure [nPa]
Figure 6.4: Comparison between the estimation of the solar wind dynamic pressure at the magnetopause crossings given by two different methods: using the balance between the dynamic pressure and the internal magnetospheric pressure and using an empirical magnetopause model. The red dots are the magnetopause crossings for which the two estimates do not differ from more than a factor of 2.
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Figure 6.5: Cassini orbits considered in the investigation of the solar wind dependence, projected in the equatorial plane (left) and in the noon-midnight meridian plane (right).
The color scale indicates the year of the orbit.
Figure 6.6: Daily number of hourly electron injections detected by MIMI/LEMMS within 2 days from the magnetopause compared to the estimation of the solar wind dynamic pressure. Upper left panel: Ram pressure estimated with the balance method. Upper right panel: Ram pressure estimated with the model method. Lower panel: Mean of the ram pressure estimates given by each method, considering only the estimates which do not differ between the two methods by more than a factor of 2.
Hence, the analysis is restricted to the pulses detected by Cassini/LEMMS within two days prior and after the magnetopause crossing.
Finally, taking into account these restrictions on the location of the QP60 events, scat-tered plots of the daily number of hourly electron injections as a function of the ram pressure estimates given by the two methods are given on the upper panels of Figure 6.6.
The bottom panel in Figure 6.6 presents the same plot with an average of the estimates given by each method and considering only the estimates which do not differ between the two methods by more than a factor of 2 (red dots in Figure 6.4). In each panel, the three colors differentiate three local time sectors: the red, green and blue events are located in the local time quadrant 03-09 LT, 09-15 LT and 15-21 LT, respectively. Only three orbits analyzed are located in the morning sector, preventing any search of correlation. For the two other local time quadrants, it appears that no linear correlation between the number of pulses per day and the ram pressure is obvious in the data.