Twenty-Seven Day Cycle The  number  of  sunspots  present  at  any  one  time is  constantly  changing  as  some  disappear  and  new  ones emerge.  As  the  sun  rotates  on  its  own  axis,  these sunspots  are  visible  at  27-day  intervals,  which  is  the approximate period for the sun to make one complete revolution.  During  this  time  period,  the  fluctuations in  ionization  are  greatest  in  the  F2  layer.  For  this reason,  calculating  critical  frequencies  for  long-distance communications  for  the  F2  layer  is  not  possible  and allowances  for  fluctuations  must  be  made. Eleven-Year Cycle Sunspots can occur unexpectedly, and the life span of   individual sunspots is variable. The ELEVEN-YEAR  SUN  SPOT  CYCLE  is  a  regular cycle  of  sunspot  activity  that  has  a  minimum  and maximum level of activity that occurs every 11 years. During  periods  of  maximum  activity,  the  ionization density  of  all  the  layers  increases.  Because  of  this, the absorption in the D layer increases and the critical frequencies  for  the  E,  F1,  and  F2  layers  are  higher. During these times, higher operating frequencies must be   used   for   long-range   communications. IRREGULAR    VARIATIONS Irregular  variations  are  just  that,  unpredictable changes  in  the  ionosphere  that  can  drastically  affect our  ability  to  communicate. The  more  common variations  are  sporadic  E,  ionospheric  disturbances, and  ionospheric  storms. Sporadic  E Irregular  cloud-like  patches  of  unusually  high ionization, called the sporadic E, often format heights near  the  normal  E  layer.  Their  exact  cause  is  not known  and  their  occurrence  cannot  be  predicted. However,  sporadic  E  is  known  to  vary  significantly with latitude. In the northern latitudes, it appears to be  closely  related  to  the  aurora  borealis  or  northern lights. The  sporadic  E  layer  can  be  so  thin  that  radio waves penetrate it easily and are returned to earth by the  upper  layers,  or  it  can  be  heavily  ionized  and extend  up  to  several  hundred  miles  into  the  ionosphere. This  condition  may  be  either  harmful  or  helpful  to radio-wave   propagation. On  the  harmful  side,  sporadic  E  may  blank  out the   use   of   higher   more   favorable   layers   or   cause additional absorption of radio waves at some frequen- cies.  It  can  also  cause  additional  multipath  problems and delay the arrival times of the rays of RF energy. On  the  helpful  side,  the  critical  frequency  of  the sporadic  E  can  be  greater  than  double  the  critical frequency of the normal ionospheric layers. This may permit  long-distance  communications  with  unusually high   frequencies.   It  may  also  permit  short-distance communications  to  locations  that  would  normally  be in  the  skip  zone. Sporadic  E  can  appear  and  disappear  in  a  short time during the day or night and usually does not occur at same time for all transmitting or receiving stations. Sudden Ionospheric Disturbances Commonly known as SID, these disturbances may occur without warning and may last for a few minutes to   several   hours.   When   SID   occurs,   long-range   hf communications  are  almost  totally  blanked  out.  The radio  operator  listening  during  this  time  will  believe his  or  her  receiver  has  gone  dead. The occurrence of SID is caused by a bright solar eruption  producing  an  unusually  intense  burst  of ultraviolet  light  that  is  not  absorbed  by  the  F1,  F2, or  E  layers.  Instead,  it  causes  the  D-layer  ionization density  to  greatly  increase.  As  a  result,  frequencies above  1  or  2  megahertz  are  unable  to  penetrate  the D  layer  and  are  completely  absorbed. Ionospheric Storms Ionospheric storms are caused by disturbances in the  earth’s  magnetic  field.  They  are  associated  with both  solar  eruptions  and  the  27-day  cycle,  meaning they are related to the rotation of the sun. The effects of ionospheric storms are a turbulent ionosphere and very  erratic  sky-wave  propagation.  The  storms  affect mostly  the  F2  layer,  reducing  its  ion  density  and causing  the  critical  frequencies  to  be  lower  than 1-11