Year: 2019
Type: article
Abstract: We explore the general properties of near-surface flows around solar active regions. Helioseismic holography is applied to HMI Dopplergrams yielding nearly 5000 flow measurements of 336 unique active regions observed by the Solar Dynamics Observatory between 2010 and 2014. Ensemble averages of the flows, over subsets of regions sorted on the basis of magnetic flux, are performed. These averages show that converging flows, with speeds of about 10 m s−1 and extending up to 10° from the active-region centers, are prevalent and have similar properties for all regions with magnetic flux above 1021 Mx. Retrograde flows are also detected, with amplitudes around 10 m s−1, which predominantly, but not exclusively, flank the polar side of the active regions. We estimate the expected contribution of these active-region flows to longitudinal averages of zonal and meridional flows and demonstrate the plausibility that they are responsible for at least some component of the time-varying global-scale flows. The reliability of our flow determination is tested using publicly available MHD simulations of both quiet-Sun convection and of a sunspot. While validating the overall methodology in general, the sunspot simulation demonstrates the presence of artifacts that may compromise quantitative flow inferences from some helioseismic measurements.
Source: The Astrophysical Journal
Author: D. C. Braun
Institution: Northwest Research Associates
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Awards: THE MERIDIONAL CIRCULATION PLAYS A CRITICAL ROLE IN THE TRANSPORT OF ANGULAR MOMENTUM AND MAGNETIC FLUX ACROSS A WIDE RANGE OF LATITUDES WITHIN THE CONVECTION ZONE AND IS A MAJOR COMPONENT OF MODELS OF THE DYNAMICS OF ROTATING STELLAR CONVECTION ZONES DYNAMOS AND THE SOLAR CYCLE. WE PROPOSE TO DETERMINE THE PHYSICAL PROPERTIES OF THE SUBSURFACE MERIDIONAL CIRCULATION THROUGH THE APPLICATION OF TWO LOCAL-HELIOSEISMIC PROCEDURES TO DOPPLERGRAMS FROM THE HELIOSEISMIC MAGNETIC IMAGER (HMI) ONBOARD THE SOLAR DYNAMICS OBSERVATORY (SDO) AS WELL AS ARCHIVED DATA FROM THE MICHELSON DOPPLER IMAGER (MDI) ONBOARD THE SOLAR AND HELIOSPHERIC OBSERVATORY (SOHO). THE TWO MAIN METHODS WE WILL EMPLOY ARE LEGENDRE FUNCTION DECOMPOSITION AND HELIOSEISMIC HOLOGRAPHY. INVERSE AND FORWARD MODELING WILL BE CARRIED OUT TO INFER DEPTH LATITUDE AND TEMPORAL PROPERTIES OF THE MERIDIONAL FLOW OVER A SPAN OF ALMOST TWO SOLAR CYCLES. THE TWO MAIN CHALLENGES TO A RELIABLE DETERMINATION OF THE MERIDIONAL FLOW - NAMELY A POOR SIGNAL-TO-NOISE RATIO AND THE PRESENCE OF SYSTEMATIC ARTIFACTS - WILL BE ADDRESSED BY 1) USING SUFFICIENTLY LONG DATASETS 2) IMPLEMENTING CONTROL MEASUREMENTS 3) PERFORMING COMPARISONS BETWEEN MULTIPLE METHODS AND INSTRUMENTS AND 4) CARRYING OUT VALIDATION TESTS WITH ARTIFICIAL DATA. OF PARTICULAR IMPORTANCE IS THE IDENTIFICATION AND REMOVAL OF SYSTEMATIC ARTIFACTS WHICH CAN MIMIC LARGE-SCALE FLOWS AND DETRIMENTALLY IMPACT THE MEASUREMENT OF THE LOW-AMPLITUDE MERIDIONAL CIRCULATION. THIS INVESTIGATION IS TIMELY IN THAT THE SUBSTANTIAL INVESTMENT OF RESOURCES INTO THE SOHO AND SDO MISSIONS IS NOW YIELDING THE DECADES-LONG DATASET REQUIRED FOR THE RELIABLE DETERMINATION OF THE PROPERTIES OF THESE FLOWS. THE PROJECT IS APPROPRIATE AND OF HIGH PRIORITY FOR THE HELIOPHYSICS SUPPORTING RESEARCH PROGRAM USING DATA FROM CURRENT NASA SPACECRAFT (SOHO AND SDO) IN COMBINATION WITH THEORY (INVERSIONS AND FORWARD MODELING) TO DIRECTLY ADDRESS THE GOALS OF THE HELIOPHYSICS DECADAL SURVEY. SPECIFICALLY THE DETERMINATION OF THE TEMPORAL AND SPATIAL CHARACTERISTICS OF THE SOLAR MERIDIONAL CIRCULATION IS CRITICAL FOR KEY SCIENCE GOAL 1: "DETERMINE THE ORIGINS OF THE SUN S ACTIVITY AND PREDICT THE VARIATIONS IN THE SPACE ENVIRONMENT." THE ROLE OF THE MERIDIONAL CIRCULATION IS SPECIFICALLY CALLED OUT IN THE DECADAL SURVEY REPORT WHICH NOTES THAT "THE DEEP PONDEROUS FLOWS THAT CARRY PATTERNS OF MAGNETIC FLUX TO THE POLES REGULATE THE SEEDING OF THE DEEP-SEATED DYNAMO THAT GENERATES SUBSEQUENT SOLAR CYCLES.", INVESTIGATING SOLAR SUPERGRANULATION WITH LOCAL HELIOSEISMIC ANALYSIS OF SDO/HMI DATA. SUPERGRANULATION IS A DISTINCT CELLULAR PATTERN OF FLOWS, DI, THIS PROJECT WILL USE SEVERAL CONTINUOUS MULTI-MONTH LONG INTERVALS OF MAGNETOGRAMS AND DOPPLERGRAMS MADE WITH THE HELIOSEISMIC AND MAGNETIC IMAGER ONBOARD THE CURRENTLY OPERATING SOLAR DYNAMICS OBSERVATORY. THESE INTERVALS WILL SAMPLE THE ENTIRE 6+ YEARS OF THE SDO MISSION SPANNING FROM A PERIOD OF EXTREME SOLAR QUIESCENCE TO THE SOLAR MAXIMUM AND INTO THE DECLINING PHASE OF CYCLE 24. THERE ARE FOUR MAIN COMPONENTS TO THE PROJECT: 1) WE WILL USE LOCAL HELIOSEISMIC TECHNIQUES INCLUDING BOTH TIME-DISTANCE AND HOLOGRAPHY MEASUREMENTS TO OBTAIN ACOUSTIC (P-MODE) TRAVEL TIMES FOR WAVES PROPAGATING THROUGH SUPERGRANULES. WE WILL EMPLOY ENSEMBLE AVERAGING OF MEASUREMENTS OVER TENS OF THOUSANDS OF SUPERGRANULES TO MINIMIZE NOISE. WE WILL MAKE TRAVEL-TIME MEASUREMENTS OF WAVES WITH A WIDE RANGE OF SKIP-DISTANCE IN ORDER TO SEPARATE CONTRIBUTIONS TO THE MEASUREMENTS OF VERTICAL AND HORIZONTAL MOTIONS WITHIN THE CONVECTIVE CELLS. 2) WE WILL INFER THE DEPTH VARIATION OF THE ENSEMBLE-AVERAGED SUPERGRANULAR FLOWS USING EXISTING AND EASILY FINE TUNED FORWARD MODELS OF THE FLOWS WHICH USE SENSITIVITY FUNCTIONS DERIVED THROUGH AN EMPIRICALLY-BASED SCHEME BASED ON THE BORN APPROXIMATION. VALIDATION OF THE MEASUREMENTS AND FLOWS WILL BE ACHIEVED THROUGH SIMULATIONS OF WAVE PROPAGATION THROUGH THE BEST MATCHED FLOWS. 3) WE WILL EXAMINE HOW TRAVEL-TIME MEASUREMENTS AND FLOWS VARY AMONGST DIFFERENT SUBSETS OF SUPERGRANULES SELECTED ACCORDING TO THE STRENGTH OF THEIR HORIZONTAL DIVERGENCE. 4) WE WILL COMPARE MEASUREMENTS AND FLOWS FOR SUPERGRANULES WITH VARYING AMOUNTS OF NEARBY MAGNETIC FLUX. THE RELATIONSHIP OF THE SUPERGRANULATION FLOWS WITH MAGNETIC FIELD WILL ALSO BE INVESTIGATED BY LOOKING AT VARIATIONS OF THEIR PROPERTIES WITH TIME OVER THE SOLAR CYCLE. UNDESIRED EFFECTS OF THE MAGNETIC NETWORK ON TRAVEL TIMES AND INFERRED FLOWS WILL BE ASSESSED THROUGH NUMERICAL SIMULATIONS., PROBING THE DEEP, PONDEROUS FLOWS: MEASUREMENT AND MODELING OF THE SOLAR MERIDIONAL CIRCULATION THE MERIDIONAL CIRCULATION PLAYS A CRITICAL ROLE IN T
OpenAlex ID: W2914217060