1. (a brief review of observations and theory)
Radio emission
from stellar flares
(a brief review of observations and theory)
Alexey Kuznetsov
Institute of Solar-Terrestrial Physics (Irkutsk, Russia)

2. Types of (flaring) radio stars
Magnetic Ap/Bp stars (CU Vir)
Interacting binaries (RS CVn, AM Her, W UMa, Algol)
Giants (FK Com)
Young stars (T Tau, young main-sequence stars)
Main sequence F/G/K stars
Main sequence M stars
Ultracool dwarfs (>M7)
... etc.
Outline of the talk
Solar-stellar connections
Radio emission mechanisms
Solar radio bursts
Radio emission of M dwarfs
Flares
Microwave range
Decimetric range
Metric range
Quiescent emission
Radio emission of F/G/K dwarfs
Young
Old
Radio Hertzsprung-Russell diagram (Gudel 2002).

3. Radio emission mechanisms
Incoherent thermal:
Thermal bremsstrahlung (free-free): requires thermal plasma. Unpolarized, broadband, Tb up to ~105 K. Example: emission from quiet solar chromosphere/corona.
Gyroresonance: requires thermal plasma in a magnetic field. Low polarization, ω ≈ 3ωB, Tb up to K. Example: microwave emission of sunspots.
Incoherent nonthermal:
Gyrosynchrotron: requires energetic electrons (~ MeV) in a magnetic field. Low polarization, broadband, Tb up to K. Example: broadband microwave emission of solar flares.
Coherent:
Require an unstable electron distribution (beam-like, loss-cone or ring-like, with energies of ~10 keV), are very sensitive to the source parameters.
Plasma: requires ωp ≥ ωB. High/low polarization, ω ≈ ωp or 2ωp, Tb up to 1015 K and above. Examples: solar type II and III bursts.
Maser: requires ωp << ωB. High polarization, ω ≈ ωB, Tb up to 1015 K and above. Example: auroral radio emission of planets.

4. Schematic spectrum of solar radio bursts (Dulk 1985)
plasma
gyrosynchrotron
closed open
Maser emission: ?

5. Solar radio fluxes at different distances
Intensity of solar radio bursts
Solar radio fluxes at different distances
1 AU
5 pc
10 pc
< 300 MHz
sfu
1-10 mJy
0.2-2 mJy
1-3 GHz
sfu
0.1-1 mJy
μJy
> 10 GHz
3×( ) sfu
μJy
7-70 μJy
Instruments for the stellar radio astronomy: VLA, Arecibo, ATCA, etc.
Sensitivity: down to ~1 mJy.
Intensity of the various components of solar radio emission (Wild et al. 1963).

6. Flares on M dwarfs – high frequencies
AD Leo (Rodono et al. 1989).
UV Cet (Smith et al. 2005).

7. Flares on M dwarfs – high frequencies
Spectral peak (slowly shifting towards lower frequencies) between 3.6 and 6 cm
EV Lac (Osten et al. 2005).
Features:
low polarization;
broad band;
high correlation with optical and/or X-ray flares;
fine structures / pulsations: ?

8. Flares on M dwarfs – high frequencies
Interpretation: incoherent gyrosynchrotron emission from high-energy electrons in flaring loops, similar to the high-frequency emission of solar flares (but with higher intensity).
Gyrosynchrotron emissivity (Dulk 1985):

9. Flares on M dwarfs – medium frequencies
UV Cet (Villadsen et al. 2016).
The bursts can have both positive and negative frequency drifts.
AD Leo (Osten & Bastian 2006).

10. Flares on M dwarfs – medium frequencies
AD Leo (Osten & Bastian 2008). The fine spectral structure cannot be interpreted by the plasma emission mechanism.

11. QPP: period ~ 0.7 s, modulation depth up to 50%
Flares on M dwarfs – medium frequencies
UV Cet (Gary et al. 1982)
QPP: period ~ 56 s
AD Leo (Bastian et al. 1990)
QPP: period ~ 0.7 s, modulation depth up to 50%

12. AD Leo, 430 MHz (Spangler et al. 1974).
Flares on M dwarfs – medium frequencies
20 cm
6 cm
X-rays
AD Leo, 430 MHz (Spangler et al. 1974).
UV Cet (Kundu et al. 1988).

13. Flares on M dwarfs – medium frequencies
Features:
high intensity;
high polarization degree (circular up to 100%, linear polarization was detected occasionally);
narrow band;
fine temporal and spectral structures (including pulsations)  high brightness temperature;
no clear correlation with optical and X-ray flares.
Interpretation: coherent emission (maser or plasma). Origin of the fine structure: modulation by small-scale inhomogeneities of plasma and/or magnetic field.
gyrosynchrotron?
maser?
AD Leo (Lang et al. 1983).

14. Decimetric radio bursts from M dwarfs – solar and planetary analogs
Solar decimetric spikes (Cliver et al. 2011).
Jovian S-bursts (Ryabov et al. 2010).
Auroral kilometric radiation of the Earth (Mutel et al. 2006).
Spikes are observed only in ~2% of the solar flares.
Maser emission mechanism is directly confirmed in the terrestrial magnetosphere.

15. Maser emission mechanism (electron-cyclotron maser instability)
Requirements:
unstable electron distribution (∂f / ∂v > 0);
strong magnetic field (ω ~ ωB);
low plasma density in the emission source (ωp << ωB);
low plasma density at the 2nd gyrolayer (to avoid reabsorption).
Plasma density in and around the AKR source (Benson et al. 1980).
Electron distribution in the AKR source (Ergun et al. 2000).

16. Maser emission mechanism (electron-cyclotron maser instability)
Alfven waves

17. Decimetric radio bursts from M dwarfs – possible interpretation
Reconnection
Alfven waves
Electric field
Plasma evacuation
Horseshoe formation
Cavity formation
Maser
How will this affect the MHD oscillations of the flaring loop?
Emission escape

18. Flares on M dwarfs – low frequencies
24 MHz
EV Lac (Abdul-Aziz et al. 1995).
No evident correlation with optical flares.
AD Leo (Boiko et al. 2012).
Multiple bursts with frequency drifts (both positive and negative) were detected.

19. Wolf 424 (Spangler & Moffett 1976).
Flares on M dwarfs – low frequencies
154 MHz
UV Cet (Lynch et al. 2017).
High circular polarization, indications of linear polarization, periodicity (with T ~ Trot).
Wolf 424 (Spangler & Moffett 1976).

20. Flares on M dwarfs – low frequencies
Features:
RFI and ionospheric effects are strong;
many surveys detected no emission (although very active stars were observed).
emission looks like a low-frequency extension of decimetric radio flares (i.e., with similar mechanism);
no clear correlation with optical and X-ray flares;
no type II and III bursts detected so far.
Possible interpretation: global magnetic fields of M dwarfs have a “confining” effect preventing formation of CMEs and fluxes of escaping energetic electrons (Odert et al. 2017).

21. M dwarfs – quiescent radio emission
UV Cet (Gudel 1994).
EV Lac (Osten et al. 2006).
Features:
high brightness temperature;
low polarization;
a typical gyrosynchrotron spectrum;
emission cannot be produced by a thermal (bremsstrahlung or gyroresonance) mechanism  nonthermal electrons are needed.
Interpretation: nonthermal gyrosynchrotron emission – either from multiple unresolved small flares or from a long-living population of energetic (up to a few MeV) electrons trapped in the global magnetic fields.

22. Young F/G/K stars – flares
3.6 cm
H II 1136 (G8), Lim & White (1995).
AB Dor (K1), Lim et al. (1994).

23. no polarization detected
Young F/G/K stars – flares
8.4 GHz, I
○ 3.6 cm, I
▪ 6 cm, I
HD (F3), Gudel et al. (1995).
no polarization detected
EK Dra (G0), Gudel et al. (1995).

24. Young F/G/K stars – quiescent radioemission
6 cm, I
AB Dor (K1), ATCA spectrum (Doyle & Metodieva).
AB Dor (K1), different rotation periods, Lim et al. (1994).

25. eps Eri (K2), Bastian et al. (2017).
Old F/G/K stars – flares
2-4 GHz
Stokes I
Stokes V
eps Eri (K2), Bastian et al. (2017).

26. Villadsen et al. (2014). No flares detected in this survey.
Old F/G/K stars – quiescent radio emission
G8.5 F9
K0.5
Villadsen et al. (2014). No flares detected in this survey.

27. Radio emission of F/G/K stars – summary
Radio emission of young (pre-main-sequence and ~zero-age main-sequence stars) seems to be qualitatively similar to that of M dwarfs.
Radio emission of older stars is more solar-like.
Sp. class
(Trot)cr, days G2
1.1 K4
3.3 M3
7.2
Two regimes of stellar dynamo: saturated (for young, fast-rotating stars) and solar-like (for older, slow-rotating stars) (Nizamov et al. 2017).