Year: 2019
Type: article
Abstract: The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur.
Source: Nature Communications
Authors: M. O. Archer, H. Hietala, M. D. Hartinger, F. Plaschke, V. Angelopoulos
Language: English
Subfield: Space and Planetary Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $5,200
Awards: SCIENCE GOALS AND OBJECTIVES MAGNETOSHEATH HIGH-SPEED JETS (HSJS) - DYNAMIC PRESSURE ENHANCEMENTS TYPICALLY OF ~1 EARTH RADIUS IN SIZE - ARE THE MOST COMMON DAYSIDE TRANSIENT. THEY IMPACT THE MAGNETOPAUSE MANY TIMES PER HOUR ESPECIALLY DURING INTERVALS OF LOW INTERPLANETARY MAGNETIC FIELD CONE-ANGLE. EMERGING FROM KINETIC PROCESSES AT THE QUASI-PARALLEL BOW SHOCK THE JETS ARE IMPORTANT MEDIATORS OF CROSS-SCALE COUPLING. UPON IMPACT THEY CAUSE MAGNETOPAUSE INDENTATIONS AND CAN COUPLE TO GLOBAL DYNAMICS BY DRIVING MAGNETOSPHERIC WAVES AND BY AFFECTING SUBSOLAR MAGNETOPAUSE RECONNECTION. ADDITIONALLY HSJS STIR MAGNETOSHEATH TURBULENCE. STRONG GRADIENTS AT JET BOUNDARIES RESULT IN SHOCK-LIKE STRUCTURES AND THIN CURRENT SHEETS CONDUCIVE OF RECONNECTION MAKING THEM POSSIBLE SITES OF PARTICLE ACCELERATION. SUCH THERMALIZATION CAN PRECONDITION THE MAGNETOSHEATH BEFORE IT MEETS THE MAGNETOPAUSE. PREVIOUS OBSERVATIONS HAVE PROVIDED INSIGHT INTO THE HSJ PROPERTIES. TWO-DIMENSIONAL (2D) HYBRID SIMULATIONS HAVE DEMONSTRATED THE FORMATION OF JETS DOWNSTREAM OF THE QUASI-PARALLEL SHOCK USING STABLE SOLAR WIND INPUTS. HOWEVER KEY AREAS REMAIN POORLY UNDERSTOOD. WE NEED TO KNOW THE 3D TOPOLOGY OF THE MAGNETIC FIELD FLOWS AND THE JETS THEMSELVES AS IT DETERMINES HOW THEY INTERACT WITH THE MAGNETOPAUSE. THE MICRO-INSTABILITIES AND PARTICLE DYNAMICS TAKING PLACE AT HSJ BOUNDARIES ARE LARGELY UNEXPLORED. IT IS ALSO UNCLEAR HOW SOLAR WIND TURBULENCE CHANGES THE HSJ OCCURRENCE AND PROPERTIES BY AFFECTING THE SURROUNDING MAGNETOSHEATH AND THE GENERATING PROCESSES AT THE BOW SHOCK. THESE QUESTIONS CAN ONLY BE ADDRESSED WITH A COMBINATION OF MULTI-POINT DATA ANALYSIS AND 3D KINETIC MODELING. DUE TO THE AVAILABILITY OF SUCH DATA FROM RECENT SPACECRAFT FLEETS AND THE COMING OF AGE OF 3D HYBRID AND KINETIC MODELS AND COMPUTATIONAL RESOURCES FOR THE FIRST TIME WE CAN MAKE SIGNIFICANT STRIDES IN THIS AREA. OUR OBJECTIVE IS TO STUDY THE ROLE OF HSJS IN DAYSIDE DYNAMICS FROM MICROSCOPIC TO GLOBAL SCALES. WE PROPOSE TO ADDRESS THE FOLLOWING SCIENCE QUESTIONS: 1) WHAT IS THE 3D TOPOLOGY OF THE MAGNETIC FIELD AND FLOWS RELATED TO HSJS? HOW DOES THIS TOPOLOGY AFFECT THE JET EVOLUTION AND INTERACTION WITH THE MAGNETOPAUSE? 2) DO JETS DRIVE SIGNIFICANT TURBULENCE RECONNECTION AND PARTICLE ACCELERATION WITHIN THE MAGNETOSHEATH? WHAT IS THEIR CONTRIBUTION TO MAGNETOSHEATH HEATING? 3) HOW DOES INPUT SOLAR WIND TURBULENCE CHANGE HSJ OCCURRENCE AND PROPERTIES? WHAT ARE THE RELATIVE CONTRIBUTIONS OF THE TRANSMITTED TURBULENCE AND HSJ-DRIVEN TURBULENCE TO MAGNETOSHEATH PROPERTIES? METHODS WE WILL RUN GLOBAL HYBRID SIMULATIONS AS WELL AS LOCAL HYBRID AND FULL PARTICLE-IN-CELL SIMULATIONS OF A SHOCK IN A BOX AND/OR JETS IMPOSED ON A BACKGROUND FLOW. WE WILL USE A THEMIS DATASET OF ~3000 HSJS (~660 SEEN SIMULTANEOUSLY BY AT LEAST TWO PROBES) AND THEIR OMNI SOLAR WIND CONDITIONS AND MULTI-SPACECRAFT CASE EVENTS FROM DIFFERENT THEMIS CONFIGURATIONS AND MMS OBSERVATIONS. WE WILL 1) ANALYZE THE JET TOPOLOGY AND EVOLUTION IN SIMULATIONS VARYING THE PLANETARY DIPOLE STRENGTH TO QUANTIFY THE EFFECTS OF SYSTEM SIZE. COMPARE WITH MULTI-POINT OBSERVATIONS. 2) ANALYZE JET BOUNDARY LAYERS IN SIMULATIONS. COMPARE WITH MMS CASE OBSERVATIONS AND ~400 SUPERMAGNETOSONIC THEMIS HSJS. 3) SIMULATE DIFFERENT LEVELS OF INPUT TURBULENCE. COMPARE WITH HSJ OBSERVATIONS DURING CORONAL MASS EJECTION SHEATH (HIGH LEVEL) AND EJECTA (LOW LEVEL) INTERVALS. RELEVANCE TO DECADAL SURVEY GOALS THE PROPOSED WORK IS RELEVANT TO ALL FOUR GOALS. THE FINDINGS WILL HELP PREDICT CHANGES IN THE SPACE ENVIRONMENT (GOAL 1) BY INCREASED UNDERSTANDING OF THE ROLE OF DAYSIDE TRANSIENTS IN GLOBAL COUPLING (GOAL 2). SAME PROCESSES ARE EXPECTED AT OTHER SHOCKS IN THE HELIOSPHERE WHEN THE SUN INTERACTS WITH THE SOLAR SYSTEM (GOAL 3). THE INVESTIGATION CONNECTS SHOCKS TURBULENCE AND RECONNECTION - FUNDAMENTAL PROCESSES OCCURRING ACROSS THE UNIVERSE (GOAL 4)., GOALS: MAGNETOHYDRODYNAMIC (MHD) FAST MODE WAVES (FMW) CAN LEAD TO ENHANCEMENTS/DEPLETIONS IN THE RELATIVISTIC ELECTRON POPULATION THAT AFFECTS SATELLITES IN GEOSTATIONARY ORBIT. THE REGION OUTSIDE THE PLASMASPHERE IS IMPORTANT FOR UNDERSTANDING FMW DYNAMICS; WAVES ORIGINATING IN THE SOLAR WIND ION FORESHOCK MAGNETOSHEATH OR OUTER MAGNETOSPHERE MUST PASS THROUGH THIS REGION BEFORE COUPLING TO INNER MAGNETOSPHERE WAVES OR INTERACTING WITH ENERGETIC PARTICLES DEEP IN THE RADIATION BELTS. PREVIOUS MODELING STUDIES MAKE TWO KEY PREDICTIONS: (1) LOW FREQUENCY FMW MOST IMPORTANT FOR RADIATION BELT INTERACTIONS OFTEN HAVE PROPAGATION CUTOFFS OUTSIDE THE PLASMASPHERE (2) FMW HAVE SIGNIFICANTLY SMALLER AMPLITUDES THAN OTHER ULF WAVE MODES. BOTH (1) AND (2) ARE CENTRAL TO OUR UNDERSTANDING OF THE ROLE OF FMW IN RADIATION BELT INTERACTIONS RELATIVE TO OTHER ULF WAVE MODES. HOWEVER THERE ARE FEW OBSERVATIONAL CONSTRAINTS TO TEST THESE PREDICTIONS; NEARLY ALL PREVIOUS STATISTICAL STUDIES OF ULF WAVE PROPERTIES LACKED IN SITU PLASMA MOMENT OBSERVATIONS A CRITICAL LINE OF EVIDENCE OFTEN NEEDED TO DISTINGUISH FMW FROM OTHER ULF WAVE MODES IN THE WARM PLASMA OUTSIDE THE PLASMASPHERE. THIS PROPOSAL IS FOCUSED ON A SINGLE OBJECTIVE DETERMINE THE PROPERTIES OF MAGNETOSPHERIC MHD FMW OUTSIDE THE PLASMASPHERE DURING DIFFERENT DRIVING CONDITIONS AIMED AT ANSWERING TWO SCIENCE QUESTIONS: HOW IS MAGNETOSPHERIC FMW ENERGY DISTRIBUTED OUTSIDE THE PLASMASPHERE AS A FUNCTION OF FREQUENCY AND DRIVING CONDITION? HOW LARGE ARE TYPICAL FMW AMPLITUDES COMPARED TO OTHER ULF WAVE MODES? THE PROPOSED STUDY WILL PROVIDE NEW OBSERVATIONS THAT BETTER CONSTRAIN THE ROLE OF FMW IN RADIATION BELT INTERACTIONS; FOR EXAMPLE THEY CAN BE USED TO DETERMINE WHETHER SOLAR WIND DRIVEN FMW PENETRATE DEEP ENOUGH INTO THE MAGNETOSPHERE TO AFFECT RADIATION BELT DYNAMICS DIRECTLY OR PLAY MORE INDIRECT ROLES BY RESONANTLY COUPLING TO STANDING ALFV N WAVES THAT IN TURN INTERACT WITH RADIATION BELT PARTICLES. MISSION DATA: OUR PRIMARY DATA FOR THESE TASKS ARE PLASMA MOMENT AND MAGNETIC/ELECTRIC FIELD OBSERVATIONS FROM NASA S TIME HISTORY OF EVENTS AND MACROSCALE INTERACTIONS DURING SUBSTORMS (THEMIS) MISSION; WE WILL USE THESE TO UNAMBIGUOUSLY IDENTIFY FMW AND RECORD THEIR AMPLITUDES. THE THEMIS SATELLITES ARE IDEAL FOR THIS STUDY AS THEY ROUTINELY PROVIDE ALL NECESSARY MEASUREMENTS WITH ROUGHLY A DECADE OF OBSERVATIONS NEAR THE MAGNETIC EQUATORIAL PLANE (WHERE FMW ENERGY IS CONCENTRATED) AND OUTSIDE THE PLASMASPHERE. WE WILL ALSO USE DATA FROM NASA S ACE AND WIND SATELLITES VIA THE OMNIWEB INTERFACE TO MONITOR SOLAR WIND CONDITIONS. TIME PERMITTING WE WILL EXAMINE DATA FROM VAN ALLEN PROBES AND MMS THOUGH THESE DATA ARE NOT REQUIRED TO ADDRESS OUR SCIENCE QUESTIONS. DATA ANALYSIS/METHODOLOGY: WE SHALL IDENTIFY FMW AS PERIODS OF WAVE ACTIVITY WITH MAGNETIC AND THERMAL PRESSURE PERTURBATIONS IN PHASE; THIS WILL REMOVE MIRROR MODE AND COMPRESSIONAL ALFVEN WAVES (POLOIDAL MODES) FROM OUR ANALYSIS. THE FMW OCCURRENCE RATE AND OTHER PROPERTIES WILL THEN BE ORDERED ACCORDING TO SPATIAL POSITION FREQUENCY AND UPSTREAM CONDITIONS OBTAINED FROM OMNIWEB. ALL STEPS IN THE METHODOLOGY HAVE BEEN TESTED AND VETTED IN PREVIOUS WORK BY THE PI AND OTHER RESEARCHERS. RELEVANCE: OUR SCIENCE OBJECTIVE ADDRESSES THE DECADAL SURVEY GOAL DETERMINE THE DYNAMICS AND COUPLING OF THE EARTHS MAGNETOSPHERE IONOSPHERE AND ATMOSPHERE AND THEIR RESPONSE TO SOLAR AND TERRESTRIAL INPUTS. IT IS ALSO RELEVANT TO THE THEMIS MISSION S SECONDARY OBJECTIVE BY DETERMINING THE EXTENT OF THE WAVE FIELDS THAT ARE PROPOSED TO ACCELERATE THE PARTICLES (THEMIS BERKELEY WEBSITE). FINALLY THE PROPOSED OBJECTIVE IS RELEVANT TO THE NASA GI MAGNETOSPHERE SUBDISCIPLINE AND SOLAR WIND MAGNETOSPHERE COUPLING SCIENCE AREA., G54188565, G5561424898, G5953495884
OpenAlex ID: W2911470357