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Rotational modulation and local time dependence of Saturn's aurora

Sarah V. Badman / JAXA International Research Fellow

 

Introduction

The Pioneer and Voyager spacecraft first detected powerful bursts of kilometric radiation from Saturn's auroral regions, the modulation of which was supposed to represent the rotation of the planet's interior. Since then, measurements made by the Ulysses and Cassini spacecraft have revealed that the Saturn Kilometric Radiation (SKR) emission periodicity varies on much shorter timescales than can be explained by changes in the planetary rotation rate, and that the emissions from the northern and southern hemispheres have their own independent emission periods. Additionally, 'planetary-period' modulations have been identified in many other magnetospheric phenomena, such as magnetic field perturbations, charged particle and energetic neutral atom populations, and ultraviolet (UV) auroral emissions. The source of this oscillatory behaviour is currently a subject of intensive research, as introduced for example in the review by Mitchell et al. (2009).

 

Figure 1.

 

Detailed analysis of the magnetic field oscillations has led to the suggestion that they are signatures of two independent auroral current systems that rotate with different periods in each hemisphere (Andrews et al., 2010). These are illustrated in Figure 1. Upward directed field-aligned currents are important for the generation of aurora at different wavelengths via the downward motion of electrons into the atmosphere. The rotating field-aligned current systems can thus influence the auroral intensity at different local times. The SKR emissions themselves are observed to be most powerful when the rotating upward current passes through the dawn sector (Andrews et al., 2010).

The location of the maximum upward current region is defined by the azimuthal direction of the rotating effective dipole (relative to local noon) in either the northern or southern hemisphere (the green and red arrows in Figure 1). This direction is called the 'magnetic phase', ƒ³M, N/S, and is equal to 0‹ when the dipole points to noon, and 90‹ when it points to dusk, etc. From Figure 1 it can be seen that the maximum upward current region is located at ƒ³M, N-90‹ in the northern hemisphere and ƒ³M, S+90‹ in the southern hemisphere at any given time.

To determine the rotational modulation of Saturn's infrared aurora, this study employs 111 observations of Saturn's H3+ aurora, at `3.6 micron wavelengths, acquired by Cassini VIMS during October 2006 to February 2009. Of these, 33 were observations of the southern aurorae and 78 were observations of the northern aurorae. For each VIMS observation, the intensities were averaged over sections 1‹ in latitude x 1 h in local time (LT). In each LT section the maximum intensity was then found between the colatitude limits of 10‹ - 25‹.

 

Example Observations

Before presenting the averaged results, we first show an example sequence of observations in Figure 2. This sequence of six observations of the northern aurora was acquired on 10 November 2006. The time at the middle of each observation is labelled on each panel. The northern magnetic phase, ƒ³M, N, at the centre time of each observation is also labelled on each panel. The solid yellow line marks the corresponding azimuth of the modelled maximum upward field-aligned current, which rotates at the defined northern hemisphere period.

 

Figure 2.

 

In panel (g) of Figure 2, Cassini Radio and Plasma Wave Science (RPWS) measurements of the SKR emitted flux density on 10 November 2006 are shown in a frequency-time spectrogram. The SKR emissions are the prominent bursts at 100s of kHz. The vertical dashed lines indicate the times of expected SKR maxima from the model (ƒ³M, N=0‹), and the labelled arrows at the top mark the times of the images. Panel (h) shows the circular polarisation of the waves, where the white colouring indicates that most of the detected SKR was emitted from the northern hemisphere, the same as the IR emission observed.

Images (a)-(c) occurred when the SKR emissions were quiet, while (e) and (f) occurred during a regular burst of SKR emission from the northern hemisphere. These were the images displaying the brightest IR emission, showing that the auroral IR and SKR intensities were correlated, and that in this case they maximised at close to the magnetic phase value (ƒ³M, N=0‹) expected from the model. In accordance with this, the most intense IR emissions were observed on the dawnside.

 

Average Results

The average intensities determined from the 111 VIMS observations are plotted as functions of LT and magnetic phase for the southern and northern hemispheres in Figure 3. The line plots in the top panels of Figures 3a and b show the LT distribution of the average intensity. These show that the H3+ emission exhibits strong local time dependences in both hemispheres, with maxima in the dawn-noon sector and minima around dusk.

 

Figure 3.

 

The colour spectrograms in Figure 3a and b show the average peak H3+ intensity as a function of LT and magnetic phase. Two cycles of magnetic phase are plotted to show the continuity of the signals. There is evidence of a linear dependence of the intensity on the magnetic phase present in both the northern and southern hemisphere data. That is, the maximum intensities were not observed at a constant magnetic phase. The expected location of the maximum upward field-aligned current is plotted by the dashed white lines on these panels. In both the northern and southern data, the maximum intensities follow the sense of this line, moving through 24 h of LT (360‹ azimuth) in each 360‹ of magnetic phase. This demonstrates that the intensity is related to the azimuthal rotation of the upward field-aligned current system in each hemisphere.

 

Discussion

The interaction of an LT-fixed, field-aligned current system with a rotating current system could explain the LT distribution of the H3+ emission intensity observed in Figure 3. As the rotating upward current sweeps through the post-dawn sector, it reinforces the LT-fixed upward current in that region leading to a maximum current (and hence auroral intensity) in this sector. When the rotating upward current region passes towards dusk, the net upward current is reduced because of the supposed quasi-static downward current present. As the rotating system continues towards midnight the downward current decreases and the net upward current increases again. This sequence explains the overall auroral intensity maxima post-dawn and the minima around dusk seen in Figures 3a and b, and the dependence on magnetic phase. Saturn's auroral emissions are thus influenced by both the solar wind interaction with the magnetosphere and the rotation of the planet.

 

References:

Badman et al. (2012), Rotational modulation and local time dependence of Saturn's infrared H3+ auroral intensity, J. Geophys. Res., 117, http://dx.doi.org/10.1029/2012JA017990.

Andrews et al. (2010), Comparison of equatorial and high-latitude magnetic field periods with north and south Saturn kilometric radiation periods, J. Geophys. Res., 115, http://dx.doi.org/10.1029/2010JA015666.

Mitchell et al. (2009), The dynamics of Saturn's magnetosphere, in Saturn from Cassini-Huygens, edited by Dougherty, M. K., Esposito, L. W., & Krimigis, S. M., doi:10.1007/978-1-4020-9217-6.