Solar Cycle 24 Intro

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(New page: Notes to the introductory talks (written by S. Christe). === Solar Activity Cycles - Past and Present by D. Hathaway (Marshall Space Flight Center) === Will there be a cycle 24? New cyc...)
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Notes to the introductory talks of the [[Solar Cycle 24]] meeting (written by S. Christe).
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Notes to the introductory talks (written by S. Christe).
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=== Solar Activity Cycles - Past and Present by D. Hathaway (Marshall Space Flight Center) ===
=== Solar Activity Cycles - Past and Present by D. Hathaway (Marshall Space Flight Center) ===

Revision as of 19:02, 14 December 2008

Notes to the introductory talks of the Solar Cycle 24 meeting (written by S. Christe).

Contents

Solar Activity Cycles - Past and Present by D. Hathaway (Marshall Space Flight Center)

Will there be a cycle 24? New cycle active regions have begun to show up the last two months. The number of spotless days is more than twice that of the last few cycles. Also Cycle 23 is the longest cycle than the last few cycles. A look at the statistics of older cycles shows out that cycle 23 is not out of the norm. On average the maximum occurs 4 years after the minimum. Statistics of cycles suggest that Cycle 24 may be small though cycles have been growing in sunspot number since 1700. Should we be worried about another Maunder Minimum (one of the grand minimum)? Grand minimum occur once every 400 years, we are currently in a grand maximum. "Prediction is very difficult especially about the future" (Niels Bohr). Forecasting an ongoing cycle is easiest about 2-3 into a new cycle (at the inflection point of the rise). Geomagnetic precursors suggest a (slightly) smaller than average cycle 24 or a much larger cycle depending on who you talk to. Geomagnetic indices (and polar field strength) are best at predicting solar cycles. Polar fields are currently weaker by a factor of 2 than previous solar minimum. Polar field indices predict a much smaller cycle 24. The first dynamo predictions suggest a cycle 24 will be much larger than cycle 23 though big cycles usually start early which has not been observed (some caveats suggest that independent confirmation of this prediction is necessary). Another dynamo model (Choudhuri 2007) predicts a small cycle 24 in keeping with geodynamic predictions (caveats also exist for this method).

Cycle 24 may be large or small! And should exist (we hope). We should know by the end of 2010.

The .pdf version of the presentation itself can be found here.

Recalibration of the Sunspot number and consequences for prediction of future activity and reconstructions of past solar behavior by L. Svalgaard (Stanford U.)

There are two real long-term sunspot numbers (group number and international number aka Wolf number). The two indices diverge 1875 making predictions difficult. People in the prediction business choose the index that works the best for their method.

The ratio of the two indices over the time of divergence show systematic changes based on changes in the human observers (~17% changes). Wolf's own sunspot (Wolf) numbers used past observations and show strange adjustements. The variation in "declination" of the geomagnetic north varies as a function of day and depends on the solar cycle. Wolf changed his sunspot numbers based on those measurements. Using this geomagnetic index it is possible to reconcile the difference between the two indices.

Data from the 19th century is currently in the process of being digitized for public distribution. The difference in the two indices is a problem which needs to be resolved for predictions to go forward.

The .pdf file containing the actual presentation can be found here.

Evolving views of solar flares, and targets for Cycle 24 by L. Fletcher (U. of Glasgow)

Looking back at cycle 18, Giovanelli (1948) had a very nice model of a solar flares. Concluded that most of the radiation of flares comes from the chromosphere (as opposed to the corona). Energy storange is in the corona. But no knowledge of nonthermal particles and CMEs.

What have we learned since then? Coronal energy is released by topological changes to coronal magnetic fields (reconnection). Up to 50% of magnetic energy released goes to nonthermal particles. Accelerated particles are present in both corona and chromosphere. The chromosphere is heated and radiates stronly and expands.

Topological changes in magnetic field create currents which are hard to work out but seems to be located near the neutral line. Halpha flare ribbons and serperatrix layers are well corolated. But how do currents change during a flare?

Energy transport in flares is mostly done by nonthermal particles. 1028 to 1029 erg per s goes into electrons. Similar energy in ions. The Accelerator is, of course, still unknown (stochastic? DC fields?).

Based on chromospheric HXRs beam models >10% of coronal electrons are accelerated and leave the corona each second. This is very large (usually refered to as the Numbers problem). But recent angular distribution measurement suggests that beams do not really exist which mean that we are using the wrong model.

Changes in the coronal magnetic structure may show up in Need to keep our eye on the chromosphere since most of the energy is released there? Can the current close in the chromosphere?

Main messages, flares are magnetic need to consider energy transport in light of this. The corona is a plasma, we must consider plasma effects such as waves and return currents etc. Most of the flare energy is made manifest in the chromosphere.

A link to the powerpoint presentation should be placed here.


Global Magnetic Field Evolution by A. Van Ballegooijen

Can deduce global fields by extrapolating photospheric fields (potential, nonpotential, MHD, force-free, PFSS). Observations suggest that fields are definitely not potential. Active regions are thought to be loops which emerge from deep twisted magnetic flux tubes. In the corona the field spreads out. Fan (2008) has simulated the rise of these flux tubes. This model suggests that active regions should be much larger (azimuthally). Something is therefore missing from this model. Convections may be the key. Fan (2003) simulated with convective flows. Speakers own work concentrated on the coupling between the corona and convection zone. Magnetic diffusion in convection zone is modified to conserve magnetic helicity. This leads to a build up of magnetic helicity in the corona which eventually leads to a loss of equilibrium. They also modeled how two active regions interact with each other. Field lines connect between active regions. The reserval of the Sun's polar magnetic field can be explained by solar differential rotation. The effects of active regions after they have decayed and submerged are not well understood. A new 2D model by speaker (in prep) seeks to understand this. Active regions tilt the underlying horizontal field consistent with Joy's law. The cumulative effects of these titls ends up reverses the poloidal field. Four different processes may be involved in the dissipation of the Sun's toroidal field.

Some conclusions, the upper convection zone is important has it stops the submergence of sheared magnetic fields. Active regions need to shed helicity by ejecting it (i.e. CMEs). Active regions effect the underlying submerged fields.

Relationship between the High and Mid latitude Solar Magnetic Field by E. Benevolenskaya given by T. Hoeksema (Stanford U.)

Transport of magnetic flux is shown by observations. In EUV, we see two sets of magnetic structures migrating. The polar field reversed in about 2000. The timescale of reversal between north and south are not the same. A more careful look at the magnetic flux shows the northern hemisphere reversing sooner as well. A comparison between MDI and WSO data shows a scale factor difference but otherwise good agreement. One particular difference, MDI shows a large south pole field. At higher latitudes (75 degrees) then there is a better correspondence with Ulysses. The discrepancy in the timing of reversal is less at high latitudes. Following small magnetic elements shows that meriodional flow is very small. At low latitudes, small magnetic elements shows equator-ward motion. Conclusions are that zonal or axisymmetrical structure of the solar cycle reveals transport of the magnetic flux between mid to high latitudes. Migration of the zonal neutral line defines the reversal of the magnetic field during the solar cycle. The transport of the magnetic energy is a complex process.


Magnetic connectivity and coronal dynamics by H. Peter (Kiepenheuer-Institut)

Studying loops on the Sun have used 1D models which are not very realistic or full 3D models but with limited resolution. The idea is to braid magnetic field lines. Currents are then generated and lead to ohmic dissipation and heating. The code used is a 3D MDH model (so-called Pencil code). A reasonable agreement of between model doppler shifts and observed shifts. Same can be said of the emission measure. 1D models are not very good at this. Looking at individual loops from this model show a spiky structure in the heating rate (maybe associated with "nanoflares"). Temperature profiles show wide variety. The heating scale height is roughly exponential with a scale height of 10 Mm (heating is concentrated in the chromosphere). Loops can also be seen to jump around which is further reason that 1D models which track a fixed loop are not reasonable. Model suggests that small loops seen in the quiet Sun are really just a projection effect and not real. The isothermal structure is much more complicated than just "traditional" loops.


The Flare/CME Connection by R. Moore (MSFC)

The bottom line is practically every CME is produced with a flare. Only seems complicated because many CME-associated flares are so weak that they are undetectable. A CME is a self-propelled magnetic bubble. The standard model is correct. The CME is produced with the flare by tether-cutting reconnection and lifts off expanding into the corona. The cartoon includes a sigmoid with a magnetic flux tube held down by an arcade. Tether-cutting reconnection begins the eruption. The escape path of the CME is determined by the surrounding field. In the outer corona, the CME moves out radially. Two testable predictions results from this theory which relates to the magnetic field at the flare being related to the angular extent of the flare and CME. Measured angular widths predict the magnetic field at source.

HInode and Flare/CME Connections: Events of 19th May, 2008 by J. Culhane (U. College London)

MDI magnetogram of 19th may active region shows a complex region and AR filaments. Halpha images show the quiescent filament which eventually join up into one filament. The active section of the filament eventually disappears and then reforms. Then come filament heating episodes, four of them in all. These episodes involve complete or practical disappearance of the Halpha features but SXR structures appear. Models for the merger and eruption of the filaments include simulations by DeVore, Antiochos, and Autanier (2005). There is no evidence for an instability. Reconnection region gives a new stable state with higher magnetic energy and smaller kinetic energy. The eruption may be kink or torus instability. A summary of the event on may 19th can be found in Veronig (2008). There is also a B9.5 flare accompanying the filament eruption. Hinode EIS observations exist and give a temperature of 11-12 MK. The flare looks like a standard model LDE. Speeds of 100 km/s are observed during the impulsive phase of the flare.

Active Region Dynamics, Flux Emergence, and Bright Points by H. Isobe (Kyoto U.)

Problems with current MHD simulations include scales are too small. Simulations are 10 times smaller than big active regions. This is a problem because the simulations are driven by the Parker instability which has a characteristic length that is small (factor of ten). Most simulations assume highly twisted flux tubes but most emerging flux regions are nearly potentially free. But large twist is necessary otherwise the simulated flux tubes do not emerge from the photosphere. Weakly twisted tubes expand horizontally and stays quiescent until the flux below is large enough so that a secondary instability begins (Acheson 79). Convection breaks up flux tubes such that no coherent structure is left. Ubuquitous small scale horizontal fields are found by Hinode (Lites 2007). Simulation by speaker shows that chromospheric reconnection prodces high frequency waves. XRT jets observed in polar region . X-ray jet are. Helical jet never seen in MDH simulations. Hall/ambipolar is significant.

A new technique for deriving electric fields from sequences of vector magnetograms by G. Fischer (U.C. Berkeley)

Electric fields determine the flux of magnetic energy and magnetic helicity. Past techniques employ local correlaction tracking to get velocity and calculate v cross B. A better method exists which consists of using the other components of the magnetic induction equation and magic. Using synthetic data, this method of finding the electric field was confirmed. Excellent recovery of the curl of E but only approximate recovery of E. A variational technique is being developed to fix this problem.

Where do Solar Filaments Form? by D. MacKay (U. of St. Andrews)

Filaments form over a wide range of latitudes. There are three different types of filaments; active region filaments, intermediate filaments, and quiescent filaments. Tang (1987) categorized filaments according to underlying magnetic polarity. In this case, filaments will be classified by their underlying magnetic polarity. The four categories are interior BR, exterior BR, interior/exterior BR, diffuse BR filaments (BR = bipolar region). Filaments observed in Halpha were overlayed onto magnetograms. Flux transport simulations were used to determine the underlying flux. It is found that 92% of filaments prefer to form in non-bipolar flux distributions consistent with Tang (1987). Only EBR showed any cycle variation. Proposed methods of formation are flux ropes emergence for small AR filaments and large IF, QF come from convergence.

Magnetic connectivity and coronal dynamics by H. Peter (Kiepenheuer-Institut)

Studying loops on the Sun have used 1D models which are not very realistic or full 3D models but with limited resolution. The idea is to braid magnetic field lines. Currents are then generated and lead to ohmic dissipation and heating. The code used is a 3D MDH model (so-called Pencil code). A reasonable agreement of between model doppler shifts and observed shifts. Same can be said of the emission measure. 1D models are not very good at this. Looking at individual loops from this model show a spiky structure in the heating rate (maybe associated with "nanoflares"). Temperature profiles show wide variety. The heating scale height is roughly exponential with a scale height of 10 Mm (heating is concentrated in the chromosphere). Loops can also be seen to jump around which is further reason that 1D models which track a fixed loop are not reasonable. Model suggests that small loops seen in the quiet Sun are really just a projection effect and not real. The isothermal structure is much more complicated than just "traditional" loops.

The Flare/CME COnnection by R. Moore (MSFC)

The bottom line is practically every CME is produced with a flare. Only seems complicated because many CME-associated flares are so weak that they are undetectable. A CME is a self-propelled magnetic bubble. The standard model is correct. The CME is produced with the flare by tether-cutting reconnection and lifts off expanding into the corona. The cartoon includes a sigmoid with a magnetic flux tube held down by an arcade. Tether-cutting reconnection begins the eruption. The escape path of the CME is determined by the surrounding field. In the outer corona, the CME moves out radially. Two testable predictions results from this theory which relates to the magnetic field at the flare being related to the angular extent of the flare and CME. Measured angular widths predict the magnetic field at source.

HInode and FLare/CME Connections: Events of 19th May, 2008 by J. Culhane (U. College London)

MDI magnetogram of 19th may active region shows a complex region and AR filaments. Halpha images show the quiescent filament which eventually join up into one filament. The active section of the filament eventually disapears and then reforms. Then come filament heating episodes, four of them in all. These episodes involve complete or partical disapearance of the Halpha features but SXR structures appear. Models for the merger and eruption of the filaments include simulations by DeVore, Antiochos, and Autanier (2005). There is no evidence for an instability. Reconnection mergion gives a new stable state with hihger magnetic energy and smaller kinetic energy. The eruption may be kink or torus instability. A summary of the event on may 19th can be found in Veronig (2008). There is also a B9.5 flare accompanying the filament eruption. Hinode EIS observations exist and give a temperature of 11-12 MK. The flare looks like a standard model LDE. Speeds of 100 km/s are observed during the impulsive phase of the flare.

Active Region Dynamics, Flux Emergence, and Bright Points by H. Isobe (Kyoto U.)

Problems with current MHD simulations include scales are too small. Simulations are 10 times smaller than big active regions. This is a problem because the simulations are driven by the Parker instability which has a characteristic length that is small (factor of ten). Most simulations assume highly twisted flux tubes but most emenerging flux regions are nearly potentially free. But large twist is necessary otherwise the simulated flux tubes do not emerge from the photosphere. Weakly twisted tubes expand horizontally and stays quiescent until the flux below is large enough so that a secondary instability begins (Acheson 79). Convection breaks up flux tubes such that no coherent structure is left. Ubuquitous small scale horizontal fields are found by Hinode (Lites 2007). Simulation by speaker shows that chromospheric reconnection prodces high frequency waves. XRT jets observed in polar region . X-ray jet are. Helical jet nevery seen in MDH simulations. Hall/ambipolar is significant.

A new technique for deriving electric fields from sequences of vector magnetograms by G. Fisher (U.C. Berkeley)

Electric fields determine the flux of magnetic energy and magnetic helicity. Past techniques employ local correlaction tracking to get velocity and calculate v cross B. A better method exists which consists of using the other components of the magnetic induction equation and magic. Using synthetic data, this method of finding the electric field was confirmed. Excellent recovery of the curl of E but only approximate recovery of E. A variational technique is being developed to fix this problem.

Where do Solar Filaments Form? by D. MacKay (U. of St. Andrews)

Filaments form over a wide range of latitudes. There are three different types of filaments; active region filaments, intermediate filaments, and quiescent filaments. Tang (1987) categorized filaments according to underlying magnetic polarity. In this case, filaments will be classified by their underlying magnetic polarity. The four categories are interior BR, exterior BR, interior/exterior BR, diffuse BR filaments (BR = bipolar region). Filaments observed in Halpha were overlayed onto magnetograms. Flux transport simulations were used to determine the underlying flux. It is found that 92% of filaments prefer to form in non-bipolar flux distribtions consistent with Tang (1987). Only EBR showed any cycle variation. Proposed methods of formation are flux ropes emergence for small AR filaments and large IF, QF come from convergence.

Solar Cycle variation of the Sun's "open" flux by N. Arge

Since Ulysses, we have learned a lot open the solar wind. The fast wind comes from the poles during solar minimum. During maximum, it is much more complicated. Cycle 23 is very similar to past ones. Ulysses found that r^2Br is independent of latitude. Any latitudes therefore provides an estimate of the total open flux from the Sun. The PFSS coronal model cannot reproduce these results but does give a measure of the total open flux which agrees well with observations. The total open flux at Earth varies by a factor of two roughly, over time. Currently at the lowest values of these observations in fourteen years. Observed flux at ULYSSES shows significant discreptancies with observed flux at the Earth. The discreptancies are maximum when ULYSSES is furthest away from the Sun. Owens et al. (2008) finds that the total flux increases with increasing R! This may be due to Bphi and Br mixing at large distances. The open fux originates from polar coronal holes near minimum and strong small regions in activity zones during maximum. Using PFSS+Schatten coronal model to help with outer corona which is more consistent with Ulysses. Comparing different input data, calculation of open flux agrees well but still significant disagreement with observed total flux. The calculated open area on the Sun varies by a factor of 2 over the solar cycle. The divergence between observed and calculated occurs when the equatorial fields get strongest. A 20% change in equatorial flux can account for discreptancy. Can achieve this by moving in source surface from 2.5 Rsun to 2.0 Rsun. When the polar fields are the primary source of open flux, the model and observations agree well. The discreptancy occurs when the equatorial flux dominates. The radial field assumption is not a valid assumpton during times of strong fields. The source surface height may vary over the solar cycle. Another problem may be that CMEs are not yet disconnected from the Sun so that the open flux is not well measured. Open flux may actually be closed. Distributed open flux in active regions may also be an answer. But, of course, observing only the front of the Sun is a significant problem as well.

Polar Plumes: What has changed since last solar minimum? by C. DeForest

Plumes arise from nearly unipolar mixed poloarity regions in the coronal hole, last about a day, and form EUV bright features that merge into bright white-light features in the mid to outer corona. Study of the magnetic structure that underly plumes is ongoing. Plume velocities are in excess of 60 km/s. Plumes might be curtain-like. Half of the fast solar wind arises from plumes. Plumes seem to be cooler than their surrounding interplume matrial, and are therefore darker in certain types of line ratio image. Recent study from stereo (curdt et al. 2008) derives the 3d structure of plumes. Plume lifetime is approximately 1 day and the cooling time is shorter than the lifetime means quasi-steady heating. Plume tomography (Barbey 2008, phd thesis) has begun and seem to show that plumes are localized bright tubes and curtains (mix of shapes) but further study is needed. Plume formation may be jet-like or aftermath of jets and evidence of wave heating now exists in observations. Wind in plumes is possible but is currently being questioned...again. Alfven waves have been detected in the plumes and may hold enough energy to drive the solar wind.

Polar Plumes and the Fast solar wind by M. Velli (JPL)

Plumes are observed in white light and EUV and associated with localized areas of mixed polarity photospheric magnetic field (Wang 2008). Long debate about the plumes and the origin of the fast solar wind. The dynamics are not clear. Plume expansion into the solar wind was first tackled by Parker (1964). Driving plumes through Kelvin-Helmholtz instability is not really reasonable. Simulations (Lionello & Velli 2009) show that plumes dynamics are more than 1D. Now on to solar wind. The fast solar wind shows a lot fine scale structure, including microstreams, meso-scale flow tubes (pressure balance structures)...maybe. Magnetic field reversals (switchbacks) observed at Ulysses probably cannot be formed low down in the corona. Polar plumes dissolve into solar wind but do not contribute significantly to the wind. There may be a spectrum of plumes such that small ones may dominate/drive the solar wind (like nanoflares for coronal heating).


Hinode mission status & Observations by T. Shimizu (ISAS/JAXA)

All telescopes are currently observing though Sun has been very quiet. Spacecraft functions are OK except for a problem in X-bank downlink which restricts to the volume of science data. Recovery from this problem is expected. The problem is related to the X-band transmiter which has caused loss of science telemetry data. Recovery from this problem means moving to S-band backup line but this means decreased downlink speed (1/16th the speed). To mitigate, more efficient data compression, removing compliomentary data, also adding more downlink stations. Downlink time will be increased by 50% or more. Reduced data from instruments still useful. Updates to the scientific operations include science planning every 2 to 3 days, proposals (HOP) are now being requested. Advice is requested by the Hinode team for how to improve observations. Currently, Hinode is in "observatory" style observations (50% of time are given to proposed observations and 50% are for core observations). Hinode has observed 3 X-class, ~5 M-class and 39 C-class flares. When energetic observations begin, Hinode observations may go into 100% core program mode. Hinode is one of the key observatories for solar cycle 24.

Hinode AR and Flare Observations by J. Cirtain (MSFC)

EIS has two different modes (raster scans or slot images which allow to cover much larger areas). NFI/SOT can make high cadence high resolution line of sight magnetograms. XRT has made a few flare observations but all are really impressive. Flare mode of operations is involved. XRT has flare mode with a specified cadence, other telescopes can repoint to the flare. XRT can store pre-flare data in a special buffer. XRT has multple filters, can use single filters or multiple filters in combination. The EIS planned observations are sit and stare at various lines, slit and slot observations. SOT can also measure stokes polarizon. SOT flare planned observations which will set in order to determine the photospheric signatures of energy storage and determine the coronal configurationo of flare sites. Can also capture the temporal evolution of filaments. The standard program definition is available online (and in the talk). External requests are possible. A plan must be submitted by 14th day of the month before your proposed observations. Requirements for proposed observations are available in the talk so are lots of useful websites.

The Halpha filter has been damaged such that light leakage from other wavelengths exists.

Operations of the IBIS Imaging spectrometer at the DST by G. Cauzzi (INAF)

Ground based observations are still important and can rival space-based observations (more flexible, no storage issues, etc). DST can match SOT resolution. DST is an imaging spectrometer at the Dunn solar telescope. FOV is 80 arcsecond. Scan time is 4s. DST is very stable. Many pre-filter passbands are available. Science done includes

Coordinated observations with SUMER and EIS. One and only observation of flares march 19th 2004 (GOES classB8, C1). Spectrum shows speeds of 60 km/s. A larger telescope is coming. Currently, ground-based are inefficient for 3 reasons, weather, scheduling, data philosophy. Need to operate ground-based telescope more like spacecraft, enter service mode (instead of PI mode). IBIS has implemented a service mode tests for the benefit of the community.

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