Ulf is a masculine name common in Scandinavia and Germany. It derives from the Old Norse word for "wolf" (úlfr, see Wulf). The oldest written record
of the name's occurrence in Sweden is from a runestone of the 11th century. The female form is Ylva. The given name Ulf was
relatively popular during the 20th century, but by the 21st century mostly fell out of fashion.
Ultra low frequency (ULF) is the ITU designation for the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In
magnetosphere science and seismology, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to
10 Hz. Frequencies above 3 Hz in atmosphere science are usually assigned to the ELF range.
Many types of waves in the ULF frequency band can be observed in the magnetosphere and on the ground. These waves represent important physical
processes in the near-Earth plasma environment. The speed of the ULF waves is often associated with the Alfvén velocity that depends on the ambient
magnetic field and plasma mass density.
This band is used for communications in mines, as it can penetrate the earth.
Ulf (or Ulf Jarl) (Ulf Thorgilsson) was a Danish earl (jarl) and regent of Denmark. Ulf was the father of King Sweyn II of Denmark and thus the
progenitor of the House of Estridsen, which would rule Denmark from 1047 to 1375, which was also sometimes, specially in Swedish sources, referred to
as the Ulfinger dynasty to honor him. 
Ulf Jarl was the son of Thorgils Sprakalägg. His brother was Eilaf, an earl of King Cnut the Great, and his sister was Gytha Thorkelsdóttir who
married Godwin, Earl of Wessex. He participated in Cnut the Great's conquest of England as one of his most trusted men. In 1015-16, he married Cnut's
sister Estrid. From c. 1024 he was appointed the Jarl of Denmark and King Cnut's appointee as regent of Denmark, which he ruled when the king was
absent. He was also the foster-father and guardian of King Cnut's son Harthacnut.
In 1026, the Swedish king Anund Jakob and the Norwegian king Olaf II took advantage of King Cnut's absence and launched an attack on the Danish in the
Baltic Sea. Ulf convinced the freemen to elect Harthacnut king, since they were discontented at Cnut's absenteeism. This was a ruse on Ulf's part
since his role as Harthacnut's guardian would make him the ruler of Denmark.
When Cnut learnt what had happened, he returned to Denmark and with Earl Ulf's help, defeated the Swedes and the Norwegians at the Battle of the
Helgeå. However, Ulf's assistance did not cause Cnut to forgive him for his own role in the coup. At a banquet in Roskilde, the two
brothers-in-law were playing chess and started arguing with each other. The next day, the Christmas of 1026, Cnut had one of his housecarls kill Earl
Ulf in Trinity Church, the predecessor of Roskilde Cathedral. However, accounts contradict each other.
In Melanesian mythology (specifically: Vanuatu), Ul is a lunar deity.