Seasonal variations in diffuse, monoenergetic, and broadband aurora

[1] The only previously established seasonal auroral variation is that of intense monoenergetic aurora, corresponding to quasi-static acceleration by geomagnetic-field-aligned electric fields. Here we investigate the separate seasonal dependence of both types of electron accelerated aurora (broadband, or wave, in addition to monoenergetic) and both ion and electron diffuse aurora. Dayside and nightside variations are separately considered, as are conditions of low and high solar wind driving. Across these many combinations, several clear patterns emerge. One is that the dayside tends to maximize precipitation in the summer and much more so for low solar wind driving. Nightside precipitation is higher in the winter and much more so for high solar wind driving. The dayside effects are strongest in number flux and stronger in diffuse aurora than accelerated aurora. The ease of ion entry through the summer cusp, along with the constraints of charge quasi-neutrality, and the rise in dayside currents in the summer hemisphere adequately explain much (perhaps all) of the dayside behavior. Nightside effects are more apparent in energy flux, with the winter/summer ratio of monoenergetic aurora being the largest: 1.70 for high solar wind driving. However, both other types of electron aurora, diffuse (1.30) and broadband (1.26), also have winter/summer energy fluxes well above unity for high solar wind driving. The nightside seasonal variation of ions is much smaller, with slightly more energy flux postmidnight in the winter but with slightly higher energy fluxes premidnight in the summer. Since the increased nightside fluxes into the winter hemisphere occur primarily under strong solar wind driving, and are much more prominent in energy flux than number flux, they likely reflect increased energization in the winter ionosphere when stronger currents are being driven into the ionosphere from the magnetosphere. Equinoctial behavior tends to lie between the summer and winter hemisphere values, but typically closer to the latter. As a result, nightside electron energy flux summed over the hemispheres is higher around equinox, simply because there is no summer hemisphere.

[1]  D. Evans,et al.  Seasonal, Kp, solar wind, and solar flux variations in long-term single-pass satellite estimates of electron and ion auroral hemispheric power , 2008 .

[2]  C. Meng,et al.  Suppression of discrete aurorae by sunlight , 1996, Nature.

[3]  S. Wing,et al.  The Cusp for High and Low Merging Rates , 2007 .

[4]  Robert L. Lysak,et al.  Feedback instability of the ionospheric resonant cavity , 1991 .

[5]  W. Calvert.  The auroral plasma cavity , 1981 .

[6]  Paul Bellan,et al.  Small Scale Alfvénic Structure in the Aurora , 2000 .

[7]  R. Elphic,et al.  FAST observations of VLF waves in the auroral zone: Evidence of very low plasma densities , 1998 .

[8]  Mike Lockwood,et al.  The cleft ion fountain , 1985 .

[9]  J. Scudder,et al.  Auroral source region: Plasma properties of the high-latitude plasma sheet , 2003 .

[10]  David G. Sibeck,et al.  Observation of IMF and seasonal effects in the location of auroral substorm onset , 2001 .

[11]  S. Petrinec,et al.  Dayside/nightside auroral X ray emission differences‐Implications for ionospheric conductance , 2000 .

[12]  C. Meng,et al.  Characteristics of the solar wind controlled auroral emissions , 1998 .

[13]  O. W. Lennartsson,et al.  The seasonal variation of auroral ion beams , 1998 .

[14]  S. Wing,et al.  Ion aurora and its seasonal variations , 2004 .

[15]  Patrick T. Newell,et al.  Hemispherical asymmetry in cusp precipitation near solstices , 1988 .

[16]  G. Khazanov,et al.  Auroral electrojet AL index and polar magnetic disturbances in two hemispheres , 2009 .

[17]  D. Klumpar,et al.  FAST observations of discrete electrostatic waves in association with down-going ion beams in the auroral zone , 2002 .

[18]  Frederick J. Rich,et al.  A nearly universal solar wind-magnetosphere coupling function inferred from 10 magnetospheric state variables , 2007 .

[19]  R. Greenwald,et al.  The role of the ionosphere in aurora and space weather , 2001 .

[20]  T. Karlsson,et al.  On low‐altitude particle acceleration and intense electric fields and their relationship to black aurora , 1997 .

[21]  Can Huang,et al.  Spectral characteristics of plasma sheet ion and electron populations during undisturbed geomagnetic conditions , 1989 .

[22]  P. Newell,et al.  Solar illumination as cause of the equinoctial preference for geomagnetic activity , 2001 .

[23]  R. Ergun,et al.  Auroral ion acceleration in dispersive Alfvén waves , 2004 .

[24]  M. Yamauchi,et al.  Energetic ion outflow from the dayside ionosphere: Categorization, classification, and statistical study , 1999 .

[25]  Robert L. McPherron,et al.  Semiannual variation of geomagnetic activity , 1973 .

[26]  C. Meng,et al.  Seasonal effects on auroral particle acceleration and precipitation , 2001 .

[27]  T. Iijima,et al.  Control of the ionospheric conductivities on large-scale Birkeland current intensities under geomagnetic quiet conditions , 1987 .

[28]  K. Laundal,et al.  Asymmetric auroral intensities in the Earth’s Northern and Southern hemispheres , 2009, Nature.

[29]  A. Kumamoto,et al.  Asymmetry of occurrence‐frequency and intensity of AKR between summer polar region and winter polar region sources , 1998 .

[30]  Patrick T. Newell,et al.  Diffuse, monoenergetic, and broadband aurora: The global precipitation budget , 2009 .

[31]  S. Wing,et al.  Central plasma sheet ion properties as inferred from ionospheric observations , 1998 .

[32]  Joseph P. Skura,et al.  OVATION: Oval variation, assessment, tracking, intensity, and online nowcasting , 2002 .

[33]  B. Mauk,et al.  Low‐altitude observations of the evolution of substorm injection boundaries , 1993 .

[34]  K. Rönnmark,et al.  The importance of solar illumination for discrete and diffuse aurora , 2005 .

[35]  C. Meng,et al.  Morphology of nightside precipitation , 1996 .

[36]  L. Blomberg,et al.  On intense diverging electric fields associated with black aurora , 1994 .

[37]  A. Ridley.  Effects of seasonal changes in the ionospheric conductances on magnetospheric field‐aligned currents , 2007 .

[38]  R. Elphic,et al.  FAST satellite observations of electric field structures in the auroral zone , 1998 .

[39]  G. Khazanov,et al.  Relationship between substorm activity and magnetic disturbances in two polar caps , 2008 .