Quantifying solar flux and geomagnetic main field influence on the equatorial thermosphere-ionosphere system for timescales complementary to satellite missions.

• Applicant: Dr. Jürgen Matzka
• Subject Area: Geophysics
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 273519587

Ionised by solar radiation, the dayside upper atmosphere - or ionosphere - is electrically conductive. Atmospheric motion in the presence of the geomagnetic field generates electric fields and currents in the ionosphere through dynamo action. These currents in turn cause magnetic fields. One of the striking features of the ionosphere, and well resolved in both satellite and ground based magnetometer measurements, is the equatorial electrojet (EEJ). This is a ribbon of high electric current density at the dayside magnetic equator in a height of about 110 km. The current is mostly eastwards directed. It is controlled by the solar flux, by the geometry and strength of the geomagnetic main field, and by tidal motion. Geomagnetic satellite constellation missions decipher in great detail the structure and dynamics of these electric currents in the ionosphere. However, processes with time scales in the order of -or in excess of- the mission duration (a few years) are difficult to quantify properly. Also, due to the restricted local time/longitudinal coverage of low-earth-orbiting satellites, localised phenomena on time scales of hours or days (like the EEJ) can only be described on a statistical, climatological basis. To overcome this observational gap, the use of data from permanent, ground-based instruments was identified within the DFG Priority Program Dynamic Earth.In this project, we want to better understand the equatorial thermosphere-ionosphere system by quantifying and studying the sensitivity of the EEJ to the geomagnetic main field and to solar flux. Our results will also lead to a better understanding of the EEJ-signal as measured by the Swarm satellites. We can achieve this, because we rediscovered handwritten tables with some 15 years worth of geomagnetic ground station data from Huancayo, a geomagnetic observatory at the magnetic equator in Peru. The rediscovered data fills in a gap in the 1960ies and 1970ies in the otherwise 90 years long time series of digital hourly mean values of the full vector geomagnetic records at Huancayo, starting at 1922. Our new data comes with two highlights: South America including Huancayo experienced the greatest relative change (10 % drop since 1968) in geomagnetic field strength of any region at the magnetic equator, allowing to study more accurately the sensitivity of the EEJ to changes in the geomagnetic main field. Secondly, it covers a solar maximum with very low sunspot numbers and the only solar maximum with recorded solar flux values (F10.7, available since 1947) comparable to the present distinctively weak solar maximum.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1788: Study of Earth system dynamics with a constellation of potential field missions
• Co-Investigator: Professorin Dr. Claudia Stolle
• Cooperation Partner: Tarique Adnan Siddiqui