The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

Cnossen, Ingrid; Wiltberger, M. J.; Ouellette, J.

doi:10.1029/2012ja017825

Cited by 36 publications

(39 citation statements)

References 40 publications

(52 reference statements)

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“…A larger offset between the geographic and invariant magnetic poles (i.e., larger tilt angle) is also expected to lead to greater variation over the course of a day. Cnossen et al [] showed that the cross‐polar cap potential minimizes when the polar cap is tilted maximally towards or away from the Sun (during equinox at 04:40 UT and 16:40 UT for a dipole position at 70°W) and maximizes in between (at 10:40 UT and 22:40 UT for the same conditions). The semidiurnal minima are deeper for a larger dipole tilt, with maxima being roughly the same for different dipole tilts [ Cnossen and Richmond , ].…”

Section: Discussionmentioning

confidence: 99%

Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry

Förster

Cnossen

2013

JGR Space Physics

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[1] The nondipolar portions of the Earth's main magnetic field constitute substantial differences between the two hemispheres. Beside the magnetic flux densities and patterns being different in the Northern Hemisphere (NH) and Southern Hemisphere (SH), also the offset between the invariant magnetic and the geographic poles is larger in the SH than in the NH. We investigated the effects of this magnetic field asymmetry on the high-latitude thermosphere and ionosphere using global numerical simulations and compared our results with recent observations. While the effects on the high-latitude plasma convection are small, the consequences for the neutral wind circulation are substantial. The cross-polar neutral wind and ion drift velocities are generally larger in the NH than the SH, and the hemispheric difference shows a semidiurnal variation. The neutral wind vorticity is likewise larger in the NH than in the SH, with the difference probably becoming larger for higher solar activity. In contrast, the spatial variance of the neutral wind is considerably larger in the SH polar region, with the hemispheric difference showing a strong semidiurnal variation. Its phase is similar to the phase of the semidiurnal variation of the hemispheric magnitude differences. Hemispheric differences in ion drift and neutral wind magnitude are most likely caused partly by the larger magnetic flux densities in the near-polar regions of the SH and partly by the larger offset between the invariant and geographic pole in the SH, while differences in spatial variance are probably just caused by the latter. We conclude that the asymmetry of the magnetic field, both in strength and in orientation, establishes substantial hemispheric differences in the neutral wind and plasma drift in the high-latitude upper atmosphere, which can help to explain observed hemispheric differences found with the Cluster/Electron Drift Instrument (EDI) and the Challenging Minisatellite Payload (CHAMP).Citation: Förster, M., and I. Cnossen (2013), Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry,

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Section: Discussionmentioning

confidence: 99%

Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry

Förster

Cnossen

2013

JGR Space Physics

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“…Cnossen et al . [] concluded that the main impact of “season” on solar wind–magnetosphere coupling was the dipole tilt (70–90%), whereas illumination contributed only 10–30%. Although this would seem to be in disagreement with the conclusions of Lyatsky et al .…”

Section: Discussionmentioning

confidence: 99%

Solar cycle effects on parallel electric field acceleration of auroral electron beams

2013

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[1] We present the occurrence frequency of downgoing auroral electron beams in magnetic local time and invariant latitude, and the dependence on solar cycle, as indicated by F10.7, on whether the ionospheric foot point of the satellite is illuminated or dark, and on the energy flux carried by the electrons. As previously reported, we find that the occurrence of electron beams peaks in the premidnight local time sector and that solar illumination at the foot point (solar zenith angle) reduces both the occurrence and energy of the electron beams. The effect of solar maximum conditions (indicated by F10.7) is almost as large as the effect of the solar zenith angle. The characteristic energy of the electron beams is dependent on the energy flux carried, in addition to both solar zenith angle and F10.7. The beam energy (and therefore the parallel potential drop) is~1.6 times higher for during solar minimum than during solar maximum for both dark and illuminated foot points. The beam energy during dark solar minimum conditions is a factor of~3 more than during sunlit minimum conditions. The "area" covered by intense aurora is also reduced during solar maximum, for both sunlit and dark conditions. There is no evidence that the statistical results are due to the fact that acceleration via parallel electric fields moves to lower latitudes during solar maximum.

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“…The modeling result by Cnossen et al [] indicated that the total potential drop along the dayside reconnection line, which maps down to the polar caps via magnetic field lines, near solstices (large dipole tilt) is lower (weaker solar wind‐magnetosphere coupling) than that near equinoxes using the same solar wind and purely southward directed IMF condition. Since flows in the magnetosheath diverge from the nose, reconnection at such a shifted reconnection line may be affected by the flow that would have a perpendicular component to the reconnection line.…”

Section: Introductionmentioning

confidence: 99%

Shift of the magnetopause reconnection line to the winter hemisphere under southward IMF conditions: Geotail and MMS observations

Kitamura

Hasegawa

Saito

et al. 2016

Geophysical Research Letters

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At 02:13 UT on 18 November 2015 when the geomagnetic dipole was tilted by −27°, the MMS spacecraft observed southward reconnection jets near the subsolar magnetopause under southward and dawnward interplanetary magnetic field conditions. Based on four‐spacecraft estimations of the magnetic field direction near the separatrix and the motion and direction of the current sheet, the location of the reconnection line was estimated to be ~1.8 RE or further northward of MMS. The Geotail spacecraft at GSM Z~1.4 RE also observed southward reconnection jets at the dawnside magnetopause 30–40 min later. The estimated reconnection line location was northward of GSM Z~2 RE. This crossing occurred when MMS observed purely southward magnetic fields in the magnetosheath. The simultaneous observations are thus consistent with the hypothesis that the dayside magnetopause reconnection line shifts from the subsolar point toward the northern (winter) hemisphere due to the effect of geomagnetic dipole tilt.

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The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

Cited by 36 publications

References 40 publications

Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry

Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry

Solar cycle effects on parallel electric field acceleration of auroral electron beams

Shift of the magnetopause reconnection line to the winter hemisphere under southward IMF conditions: Geotail and MMS observations

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