1. A SOLAR FLARE is defined as a
sudden, rapid, and intense variation in brightness.
A solar flare occurs when magnetic energy that has been built up in the solar atmosphere is suddenly released.
Radiation is emitted across the spectrum -- radio, visible, x-ray, gamma-rays.
The amount of energy released is equivalent to millions of 100-megaton hydrogen bombs exploding at the same time.
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

2. In solar flares electrons are both
heated to high temperatures, and accelerated
The electrons are thought to be accelerated by the collapse of stretched magnetic field lines high above the solar surface (``magnetic reconnection'').
Energy is released when intense magnetic fields relax back to a quiescent state (much like a stretched rubber band). The energy is transmitted to the particles trapped on the filed lines.
The accelerated electrons heat up the thermal plasma in the loop directly, and indirectly by “chromospheric evaporation”. The soft (thermal) x-rays seen by TRACE reflect this heating.
The hard X-rays from the base of the active region are ``bremsstrahlung'', or ``braking radiation'', caused by
electrons slamming into the dense gases at the bottom of the corona.
This heated chromospheric gas rises up (“chromospheric evaporation”) and also heats the thermal plasma in the loop.
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

3. Bremsstrahlung Radiation
High-energy electrons are decelerated through attraction by positively-charged “low-energy” ions. When electrons are decelerated, they give off radiation called “bremsstrahlung” (or “braking”) radiation, usually in the form of “hard” x-rays, i.e., energies of order keV
The type of radiation given off by the heated “thermal” (10-30 million K)
plasma is different, consisting of “soft” x-rays (typically 1-10 keV).
Some thermal bremsstrahlung from very hot thermal plasma
(> 30 million K) is also emitted
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

4. Observations of hard x-rays (10-100 keV) allow us to study
There are typically three stages to a solar flare (each lasting from ~seconds to ~1 hour).
precursor stage: release of magnetic energy is triggered. Soft x-ray emissions.
impulsive stage: protons and electrons are accelerated to energies exceeding 1 Mev; radio waves, hard x-rays, and
gamma rays are emitted.
decay stage: gradual build up and decay of soft x-rays.
Observations of hard x-rays ( keV) allow us to study
the accelerated electrons and the hottest plasma in flares
Observations of soft x-rays (1-10 keV) allow us to study
the thermal plasma component
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

6. The relationship between the nonthermal (accelerated) electrons and the hottest thermal electrons can be studied by observing the time evolution of both components during a flare. Likewise, the relationship between these energetic components and somewhat cooler thermal plasma can be studied by comparing the hard x-ray observations with the evolution of the soft x-ray emission.
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

7. Optical Classification of Flares
The optical (as seen in Hydrogen-alpha light) classification of a flare is made using a two-character designation. For example, a 1B designation indicates a ``brilliant” intensity flare covering a corrected area between 100 and 249 millionths of the solar hemisphere.
FLARE BRIGHTNESS
CATEGORIES:
F: FAINT
N: NORMAL
B: BRILLIANT
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

8. Frequency of Optical Solar Flares During Cycles 20-21
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

9. X-Ray Classification of Flares
The most common x-ray index is based on the peak energy flux of the flare in the 1 to 8 Å x-ray band measured by geosynchronous satellites. These measurements must be made from space, since the Earth’s atmosphere absorbs all solar x-rays before they reach the Earth’s surface.
Classification X-Ray Flux
(ergs/cm2-sec) C M X
The left categories are broken down into nine subcategories based on the first digit of the actual peak flux. For example, a peak flux of 5.7 x 10-2 ergs/cm2-sec is reported as a M5 soft x-ray flare.
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

10. Bastille Day Event July 14, 2000
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

11. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

12. CORONAL MASS EJECTIONS (CMEs)
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

13. ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

14. CME Rate Solwind (1979-1984) SMM (1984-1989) SOHO (1996-2002)
27d Average 2800MHz Solar Flux
----- (Max=254)
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

15. CME Latitude Distributions
SOHO LASCO
CME Latitude Distributions
1996
2000
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

16. How are Flares and CME's Related?
The need to release built-up magnetic field energy leads to both
flares and CMEs.
CMEs involve the eruption of a confining magnetic field, but only a special class
of flares called Solar Proton Events (SPEs) involve the breaking apart of
magnetic field lines and spewing out of
highly energetic protons.
There is good association between CMEs and Long-Duration-Event (LDE) soft X-ray flares ( or gradual flares).
Sometimes flares are categorized as impulsive (more compact; last minutes; occurring low in the corona) and gradual.
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

17. solar energetic proton events ever recorded
The Bastille-day flare was ‘X-class’ and accompanied by one of the largest
solar energetic proton events ever recorded
Confirm this as a X-class flare by converting the Watts/m^2 to erg/cm^2/s.
c3714
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

18. Solar Physics in the Media
The great solar storms of October/November 2003
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

19. Sun’s Radiation and the Upper Atmosphere

20. 81-day Mean Density Normalized to 390 km
Derived from Precise Orbit Determination of MGS
(370 x 437 km orbit; perigee near -40º to -60º latitude, 1400 LT)
81-day mean
F10.7 solar
flux at 1 AU
81-day mean
F10.7 solar
flux at Mars
81-day mean density
Note: Each density determination is made over 3-5 Mars days, and is a longitude average, so there
is no possibility to derive longitude variability, e.g., as seen in MGS accelerometer data.
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

21. Least-Squares Fit to Exosphere Temperature Derived from
Observed Densities and DTM-Mars (Lemoine and Bruinsma, 2002)
zonal mean
dust optical
depth ±30o
latitude avg.
Fit for density (10-18 cm-3):
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere

22. Earth-Mars Comparative Response to Long-Term Changes in Solar flux
Ls variation
removed
Earth - msise90
40o latitude
Mars - MGS
Mars is ~36% - 50% as responsive to solar flux received at the planet,
compared to Earth, consistent with Forbes et al. (2006)
Earth-Mars Comparative Response to Long-Term Changes in Solar flux
ASEN 5335 Aerospace Environments -- The Sun & Intro to Earth’s Upper Atmosphere