Are Dawn Storms Jupiter's Auroral Substorms?

Abstract

The specificity of the dawn storms among the various auroral morphologies at Jupiter was recognized as soon as the first high resolution ultraviolet (UV) images of the aurorae on Jupiter became available (Gérard et al., 1994). As observed from the Hubble Space Telescope (HST), having only access to the Earth-facing side of the aurora, they consist of a thickening and a major enhancement of the brightness of the dawn arc of the main auroral emission (main oval). They seem to last for at least 1-2 h (Ballester et al., 1996), but given the typical length of HST sequences is ∼45 min, HST could not provide a complete and uninterrupted view of the process. Dawn storms are also characterized by clear signatures of methane absorption, indicating that the charged particles causing them can precipitate deep below the methane homopause, with energies up to 460 keV (Gustin et al., 2006) in the case of electrons. Based on the large HST observation campaign carried out in 2007, dawn storms appeared rare (3 cases out of 54 observations) and occurred independently from the state of the solar wind (Nichols et al., 2009). However, the dawn storm observed Abstract Dawn storms are among the brightest events in the Jovian aurorae. Up to now, they had only been observed from Earth-based observatories, only showing the Sun-facing side of the planet. Here, we show for the first time global views of the phenomenon, from its initiation to its end and from the nightside of the aurora onto the dayside. Based on Juno's first 20 orbits, some patterns now emerge. Small short-lived spots are often seen a couple of hours before the main emission starts to brighten and evolve from a straight arc to a more irregular one in the midnight sector. As the whole feature rotates dawnward, the arc then separates into two arcs with a central initially void region that is progressively filled with emissions. A gap in longitude then often forms before the whole feature dims. Finally, it transforms into an equatorward-moving patch of auroral emissions associated with plasma injection signatures. Some dawn storms remain weak and never fully develop. We also found cases of successive dawn storms within a few hours. Dawn storms thus share many fundamental features with the auroral signatures of the substorms at Earth, despite the substantial differences between the dynamics of the magnetosphere at the two planets. Plain Language Summary Polar aurorae are a direct consequence of the dynamics of the plasma in the magnetosphere. The sources of mass and energy differ between the Earth's and Jupiter's magnetospheres, leading to fundamentally distinct auroral morphologies and very different responses to solar wind variations. Here, we report on the imaging of all development stages of spectacular auroral events at Jupiter, called dawn storms, including, for the first time, their initiation on the nightside. Our results reveal surprising similarities with auroral substorms at Earth, which are auroral events stemming from explosive magnetospheric reconfigurations. These...

Simultaneous Observation of an Auroral Dawn Storm With the Hubble Space Telescope and Juno

“…DOY, days of year; JADE, Jovian Auroral Distributions Experiment; JEDI, Jupiter Energetic particle Detector Instrument. As in Figure 4, the yellow shaded region shows the HST observation period during which the dawn storm onset was observed, although the dawn storm is expected to persist for several hours beyond this observation period (Bonfond et al, 2021;Kimura et al, 2017). DOY, days of year.…”

“…This suggests electrons precipitating in the region penetrate to greater depths in the upper atmosphere, requiring higher electron energies than are typically calculated for the main auroral emission (∼100 keV compared to 460 keV for bright dawn emissions (Cowley & Bunce, 2001; Gustin et al., 2006)). Additionally, the association with local time implies a relationship with the solar wind, but observations performed during the New Horizons flyby in 2007 (Clarke et al., 2009; Nichols et al., 2009) and later with Juno (Bonfond et al., 2021) found no correlation between solar wind conditions and the occurrence of dawn storms.…”

Jupiter's Double‐Arc Aurora as a Signature of Magnetic Reconnection: Simultaneous Observations From HST and Juno

“…A recent study by Bonfond et al. (2021) has shown a Jupiter auroral process very similar to terrestrial pseudo‐breakup. All in all, the intensification of the poleward auroral in the planetary ionosphere is a common feature of magnetic reconnection, and the present study provides direct evidence of such a process at Jupiter.…”

“…10.1029/2021GL093964 from 16:26 to 16:41 UT, which is significantly larger than the typical Alfvén speed (less than 300 km/s which corresponds to moving <4 R J within 15 min) in Jupiter's magnetosphere (Bonfond et al, 2021;Kim et al, 2020). Based on the mapping, it is therefore unlikely that the change of auroral source location in the magnetosphere is the major reason for the equatorward motion of auroral arcs.…”

“…Although terrestrial substorms are traditionally caused by magnetospheric current disruption (Lui, 1996), reconnection may directly drive pseudo-breakup substorms (Pu et al, 2010). A recent study by Bonfond et al (2021) has shown a Jupiter auroral process very similar to terrestrial pseudo-breakup. All in all, the intensification of the poleward auroral in the planetary ionosphere is a common feature of magnetic reconnection, and the present study provides direct evidence of such a process at Jupiter.…”