Hemispheric and seasonal variations in the cold plasma outflow source region: polar cap ionosphere electron density at 350&#8211;500 km
Authors: Hatch Spencer , Haaland Stein , Laundal Karl Magnus , Jørgensen Therese Moretto , Yau Andrew , Bjoland Lindis , Reistad Jone Peter , Ohma Anders , Oksavik Kjellmar 
Published: March 2020

&lt;p&gt;&lt;span&gt;The polar cap &lt;/span&gt;&lt;span&gt;ionosphere &lt;/span&gt;&lt;span&gt;(here &lt;/span&gt;&lt;span&gt;defined as the region above 80&amp;#176; magnetic latitude&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; is the primary source region of cold plasma outflows &lt;/span&gt;&lt;span&gt;observed in the magnetospher&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;The &lt;/span&gt;&lt;span&gt;two &lt;/span&gt;&lt;span&gt;factors controlling cold plasma outflows are &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;availability of plasma in the &lt;/span&gt;&lt;span&gt;polar cap ionosphere,&lt;/span&gt;&lt;span&gt; and transport &lt;/span&gt;&lt;span&gt;from the ionosphere to the magnetosphere&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Some statistical &lt;/span&gt;&lt;span&gt;studies have &lt;/span&gt;&lt;span&gt;indicated &lt;/span&gt;&lt;span&gt;that &lt;/span&gt;&lt;span&gt;the former of these two factors, &lt;/span&gt;&lt;span&gt;availability &lt;/span&gt;&lt;span&gt;of cold plasma&lt;/span&gt;&lt;span&gt;, is &lt;/span&gt;&lt;span&gt;the limiting factor&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;We use &lt;/span&gt;&lt;span&gt;15 years of &lt;/span&gt;&lt;span&gt;electron density &lt;/span&gt;&lt;span&gt;measurements made by Swarm and CHAMP spacecraft, &lt;/span&gt;&lt;span&gt;corrected for variations in observation altitude and solar activity,&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;investigate how variations in solar wind driving &lt;/span&gt;&lt;span&gt;and &lt;/span&gt;&lt;span&gt;local hemispheric season affect the polar cap ionosphere electron density &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;&lt;em&gt;.&lt;/em&gt;&lt;/span&gt; &lt;span&gt;We show that the dependence of &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; on the &lt;/span&gt;&lt;span&gt;&lt;em&gt;B&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;y&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; component of the interplanetary magnetic field is apparently antisymmetric in the two hemispheres, that &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;statistically decreases with decreasing &lt;/span&gt;&lt;span&gt;&lt;em&gt;Dst&lt;/em&gt;&lt;/span&gt;&lt;span&gt; index (i.e., increasing geomagnetic activity) &lt;/span&gt;&lt;span&gt;and that &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; is apparently&lt;/span&gt; &lt;span&gt;insensitive to the &lt;/span&gt;&lt;span&gt;&lt;em&gt;AE&lt;/em&gt;&lt;/span&gt;&lt;span&gt; index&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;W&lt;/span&gt;&lt;span&gt;e &lt;/span&gt;&lt;span&gt;also&lt;/span&gt;&lt;span&gt; show that &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; distributions around March and September equinoxes display &lt;/span&gt;&lt;span&gt;weak &lt;/span&gt;&lt;span&gt;evidence of &lt;/span&gt;&lt;span&gt;hemispheric asymmetry. &lt;/span&gt;&lt;span&gt;We show that during local summer, observed &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; distributions under high solar wind driving conditions are relatively lower than &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; distributions under low solar wind driving conditions. During local winter the reverse is true, with &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; distributions under low solar wind driving conditions being relatively lower than &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; distributions under high solar wind driving conditions. &lt;/span&gt;&lt;span&gt;Thus solar wind driving and seasonal effects may apparently both constructively and destructively interfere. &lt;/span&gt;&lt;span&gt;Altitude variation &lt;/span&gt;&lt;span&gt;in &lt;/span&gt;&lt;span&gt;Swarm and CHAMP &lt;/span&gt;&lt;span&gt;&lt;em&gt;N&lt;/em&gt;&lt;/span&gt;&lt;sub&gt;&lt;span&gt;&lt;em&gt;e&lt;/em&gt;&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;measurement&lt;/span&gt;&lt;span&gt;s &lt;/span&gt;&lt;span&gt;is accounted for via an empirical scale height derived from 1687 conjunctions between Swarm B and either Swarm A or Swarm C during 2013&amp;#8211;2019. The approximately linear dependence of &lt;em&gt;N&lt;/em&gt;&lt;sub&gt;&lt;em&gt;e&lt;/em&gt;&lt;/sub&gt; on F10.7&lt;/span&gt;&lt;span&gt; measurements&lt;/span&gt;&lt;span&gt; is also &lt;/span&gt;&lt;span&gt;accounted for&lt;/span&gt;&lt;span&gt;. Swarm &lt;em&gt;N&lt;/em&gt;&lt;sub&gt;&lt;em&gt;e&lt;/em&gt;&lt;/sub&gt; measurements are additionally corrected using the Lomidze et al. (2018) calibrations. &lt;/span&gt;&lt;/p&gt;