Radio Emission from the Sun: Recent Advances at High Frequencies Bin Chen New Jersey Ins�Tute of Technology the Radio Sun

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Radio emission from the Sun: Recent advances at high frequencies Bin Chen New Jersey Instute of Technology The Radio Sun

Credit: S. White 2 Solar Radio Emission • Produced by diﬀerent sources via a variety of emission processes • Provides rich diagnoscs for both thermal plasma and nonthermal electrons

Bremsstrahlung Gyromagnec Radiaon Coherent Radiaon

− − − e e e γ γ

Nucleus γ B ² Energec electrons ² Energec electrons ² Thermal plasma ² Magnec ﬁeld ² Background plasma ² Thermal plasma 3 74 SOLAR AND SPACE WEATHER RADIOPHYSICS

From D. Gary

Figure 4.1. Characteristic radio frequencies for the solar atmosphere. The upper-most characteristic frequency at a given frequency determines the dominant emission mechanism. The plot is meant to be schematic only, and is based on a model of temperature, density, and magnetic ﬁeld as follows: The density is based on the VAL model B (Vernazza et al. 1981), extended to 105 km by requiring hydrostatic equilibrium, and then matched by a scale factor to agree with 5 the Saito et al. (1970) minimum corona model above that height (the factor of 5 was chosen to £ give 30 kHz as the 1 AU plasma frequency). Temperature was based on the VAL model to about 105 km, then extended to 2 106 K by a hydrostatic equilibrium model. The temperature is then £ taken to be constant to 1 AU. The magnetic ﬁeld strength was taken to be the typical value for 1.5 active regions given by Dulk & McLean (1978), B =0.5(R/R 1) . For the ∫(øÆ =1) Ø °2 curve, a scale height L is needed. We used L = H0(T/T0)(R/R ) where H0 =0.1R and 6 Ø Ø T0 =2 10 K. Near the Sun, the curves apply to active regions. £

third harmonic ∫ =3∫B. Figure 1 shows that the 3∫B line lies above the øÆ =1 level down to 1–2 GHz, and extends up in frequency to 20 GHz—both of which agree well with the observed range. During bursts, gyroemissionª is more typically at ∫ = 10∫B, from which we see that gyroemission during bursts can extend to 800–900 MHz in the decimetric range. At mm wavelengths 100 GHz, bursts can be dominated by either free-free or gyroemission, dependingª on the number and energy of emitting particles. Outside of ﬂares, the emission above 20 GHz is entirely due to free-free emission. Solar Radio Observing Techniques: Dynamic Spectroscopy FFT Machine Antenna +

Dynamic Spectrum: In most cases, our spectral resoluon and cadence are not limited by sensivity, but by the instrumentaon capability Tun et al. 2015 5 Solar Radio Observing Techniques: Radio Synthesis Imaging

Radio Interferometer

Credit: S. White 6 When Imaging Meets Dynamic Spectroscopy

2012 Mar 10 M8.4 ﬂare D A VLA “vector” X-ray light curve dynamic spectrum B AIA 335 @ 10-Mar-2012 17:57:27 500 C VLA cross-power dynamic spectrum 450 EUV, X-ray, SXR Source and radio 400

Y (arcsecs) E images 350 HXR FP HXR FP VLA “vector” VLA burst source @ 1.2 GHz VLA GS source @ 1.9 GHz 300 dynamic spectrum RHESSI 12-15 keV RHESSI 30-70 keV SDO/AIA 335 Å 250 300 350 400 450 500 550 X (arcsecs)

AIA 171 @ 10-Mar-2012 20:00:00 500

450

400 Y (arcsecs) 350

VLA burst source @ 1.2 GHz VLA GS source @ 1.9 GHz 300 RHESSI 12-15 keV RHESSI 30-70 keV 250 300 350 400 450 500 550 X (arcsecs) New generaon radio facilies: Jansky VLA • General purpose radio observatory Karl G. Jansky Very Large Array • Microwave range: 1–8 GHz (currently available for solar observing)

• Probes low solar corona (<~1.15 Rsun). Great for solar ﬂare and acve region science. v 27 25-meter antennas v ~20”/GHz for C conﬁguraon v Instantaneous bandwidth up to 2 GHz Solar observing started in late 2011 v Full Stokes polarizaon v Up to 50 ms and 1 MHz resoluon

8 New generaon radio facilies: EOVSA

Expanded Owens Valley Solar Array • Solar-dedicated radio observatory • Microwave range 1–18 GHz (now 2.5–18 GHz due to RFI)

• Probes low corona (<~1.15 Rsun). Great for solar ﬂare and acve region science

v 13 2.1-m antennas + 1 27-m antenna for calibraon v Max baseline 1.56 km (typically ~60”/GHz) v • Located at OVRO/Caltech, Full Stokes correlaon operated by NJIT v Sweeps 1–18 GHz in 1 second • Fully commissioned in 2017 9 New generaon radio facilies: ALMA

Atacama Large Millimeter Array • General purpose radio observatory • Bands 3 and 6 currently available for solar observing (~100 GHz & 230 GHz, Bands 7 & 9 being commissioned) • Probes solar chromosphere (~1–1.003

Rsun). Great for observing detailed thermal structures in chromosphere v ~50 antennas for INT and 3 antennas Solar observing started in 2016 (Cycle 4) for total-power fast scan v Sub-arcsecond spaal resoluon!

10 Solar ﬂare and Coronal Mass Ejecons

Flare signatures near the surface CME (white light)

From T. Basan Uniﬁed ﬂare-CME model

Lin et al 2004, ApJ, 602, 422 Outstanding Quesons q When, where, and how do major space weather drivers such as solar ﬂares and coronal mass ejecons occur? q Solar ﬂares and CMEs are excellent laboratories to study catastrophic magnec energy release processes that also occur on other stars v Where and how does magnec energy store and release? v How are charged parcles accelerated to relavisc speeds? v How is plasma heated to mulple millions of degrees? v How does energy transport in the highly-coupled solar atmosphere?

13 Recent Examples from EOVSA: Imaging ﬂare loops ﬁlled with energec electrons

X8.1 flare on 2017 Sep 10 Recent Examples from EOVSA: Imaging flare loops filled with energec electrons Spaally Resolved Gyrosynchrotron Spectra

Recent Examples from EOVSA: Mapping Solar Acve Regions o Solar acve regions are the source for all major solar acvies o Measuring B ﬁeld in the corona remains a major challenge at all other wavelengths, but readily accessible from radio gyroresonance radiaon o Requires wide frequency coverage to sample B ﬁeld in acve regions (100s to 1000s G)

Background: SDO/HMI magnetogram (photosphere) Contours: EOVSA radio map at ~5 GHz (corona) Recent Examples from EOVSA: Mapping Solar Acve Regions

From D. Gary

6 “Isogauss” gyroresonance layers: f sf ce == .82 ×10 sB 18

Recent Examples from EOVSA: Mapping Solar Acve Regions

B ~ 350 G 402 G 415 G 472 G 530 G 594 G Assuming s=3 From D. Gary Examples of Solar Studies with JVLA

NRAO Press Release 2015 Dec 3 • Mapping solar ﬂare terminaon shock – Chen et al. 2015, Science, 350, 1238

• Tracing fast electron beams NRAO 2013 Science Highlights – Chen et al. 2017, in prep – Chen et al. 2013, ApJL, 763, 21

20 Solar Flare Terminaon Shock

q TSs suggested as one mechanism for accelerang electrons in ﬂares q However, solid observaonal Reconnecon evidence remains elusive

oulow

Terminaon Shock Reconnected Loops

21 Radio Emission at a Terminaon Shock

Reconnecon Oulow

Vdownflow > Cms e- Accelerated Electrons Terminaon Shock Turbulence/Fluctuaons - n1 e n2 Langmuir waves: f ~ n1/2 f1 Flux f2

frequency 22 Radio and HXR source at the front of reconnecon downﬂows Erupon

VLA 1-2 GHz

Reconnecon

Downﬂows

LT Radio (and HXR) Hot Loops

FP Radio Driing structure consisng of numerous short-lived, narrowband coherent radio bursts

23 Dynamic shock surface outlined

Radio Dynamic Spectrum Shock Disrupted Frequency

LT HXR

Shock evoluon MHD simulaon Time in 1-s cadence (by C. Shen @ CfA) Main results: • First convincing observaonal idenﬁcaon of a solar ﬂare terminaon shock • Demonstrated its role in accelerang electrons Chen et al. 2015, Science, 350, 1238 Examples of Solar Studies with JVLA

NRAO Press Release 2015 Dec 3 • Mapping solar ﬂare terminaon shock – Chen et al. 2015, Science, 350, 1238

• Tracing fast electron beams NRAO 2013 Science Highlights – Chen et al. 2017, in prep – Chen et al. 2013, ApJL, 763, 21

25 Dm-λ type III radio bursts from fast electron beams

1/2 f ~ 1 or 2 fpe = 9ne kHz

9 10 -3 f ~ 1 GHz ne ~ 10 -10 cm

26 Snapshot of a Beam Trajectory

High-resoluon Channel-by-channel Dynamic spectrum spectral imaging Electron beam trajectory Lower

Higher Frequency

Chen et al. 2013, ApJL, 763, 21 Dynamic spectrum Colored in Freq Colored in Time Colored in Time

Seconds since 19:10:27:325

Seconds since 19:10:36:425 28 Erupng Jet-Reconnecon Scenario Radio, EUV, and X-ray Observaons

AIA EUV 193 Å

EUV Jet 0 s Electron Beams 1 s

Reconnecon null point

Reconnected ﬂare arcades 5,000 km x-ray 4–8 keV – + –

Pinpoinng the magnec reconnecon/electron acceleraon site Chen et al., in prep Recent Examples from ALMA: Imaging detailed structures in chromosphere

Thermal structure & magnec ﬁeld (Stephen’s talk) Credit: ALMA (NRAO/ESO/NAOJ) Recent Examples from ALMA: Probing dynamics in chromosphere Concluding Remarks

• Solar radio astronomy has entered a new era, thanks to new instrumentaon that oﬀers broadband dynamic spectral imaging, opening up new opportunies for solar physics and space weather research • Provides detailed studies of magnec energy storage and release, parcle acceleraon, emission processes, as well as structure, dynamics, and magnec ﬁeld of the solar atmosphere Thank you